content: Split out VideoCaptureMachine from DesktopCaptureDeviceAura

content/browser/media/capture/desktop_capture_device_aura.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/media/capture/desktop_capture_device_aura.h"
 
 #include "base/logging.h"
-#include "base/metrics/histogram.h"
 #include "base/timer/timer.h"
-#include "cc/output/copy_output_request.h"
-#include "cc/output/copy_output_result.h"
-#include "content/browser/compositor/image_transport_factory.h"
+#include "content/browser/media/capture/aura_window_capture_machine.h"
 #include "content/browser/media/capture/content_video_capture_device_core.h"
-#include "content/browser/media/capture/desktop_capture_device_uma_types.h"
-#include "content/common/gpu/client/gl_helper.h"
 #include "content/public/browser/browser_thread.h"
-#include "content/public/browser/power_save_blocker.h"
-#include "media/base/video_capture_types.h"
-#include "media/base/video_util.h"
-#include "skia/ext/image_operations.h"
-#include "third_party/skia/include/core/SkBitmap.h"
-#include "ui/aura/client/screen_position_client.h"
-#include "ui/aura/env.h"
 #include "ui/aura/window.h"
-#include "ui/aura/window_observer.h"
-#include "ui/aura/window_tree_host.h"
-#include "ui/base/cursor/cursors_aura.h"
-#include "ui/compositor/compositor.h"
-#include "ui/compositor/dip_util.h"
-#include "ui/compositor/layer.h"
-#include "ui/gfx/screen.h"
 
 namespace content {
 
-namespace {
-
-int clip_byte(int x) {
-  return std::max(0, std::min(x, 255));
+DesktopCaptureDeviceAura::DesktopCaptureDeviceAura()
+    : machine_(new AuraWindowCaptureMachine()),
+      core_(new ContentVideoCaptureDeviceCore(
+          scoped_ptr<VideoCaptureMachine>(machine_))) {
 }
 
-int alpha_blend(int alpha, int src, int dst) {
-  return (src * alpha + dst * (255 - alpha)) / 255;
-}
-
-// Helper function to composite a cursor bitmap on a YUV420 video frame.
-void RenderCursorOnVideoFrame(
-    const scoped_refptr<media::VideoFrame>& target,
-    const SkBitmap& cursor_bitmap,
-    const gfx::Point& cursor_position) {
-  DCHECK(target.get());
-  DCHECK(!cursor_bitmap.isNull());
-
-  gfx::Rect rect = gfx::IntersectRects(
-      gfx::Rect(cursor_bitmap.width(), cursor_bitmap.height()) +
-          gfx::Vector2d(cursor_position.x(), cursor_position.y()),
-      target->visible_rect());
-
-  cursor_bitmap.lockPixels();
-  for (int y = rect.y(); y < rect.bottom(); ++y) {
-    int cursor_y = y - cursor_position.y();
-    uint8* yplane = target->data(media::VideoFrame::kYPlane) +
-        y * target->row_bytes(media::VideoFrame::kYPlane);
-    uint8* uplane = target->data(media::VideoFrame::kUPlane) +
-        (y / 2) * target->row_bytes(media::VideoFrame::kUPlane);
-    uint8* vplane = target->data(media::VideoFrame::kVPlane) +
-        (y / 2) * target->row_bytes(media::VideoFrame::kVPlane);
-    for (int x = rect.x(); x < rect.right(); ++x) {
-      int cursor_x = x - cursor_position.x();
-      SkColor color = cursor_bitmap.getColor(cursor_x, cursor_y);
-      int alpha = SkColorGetA(color);
-      int color_r = SkColorGetR(color);
-      int color_g = SkColorGetG(color);
-      int color_b = SkColorGetB(color);
-      int color_y = clip_byte(((color_r * 66 + color_g * 129 + color_b * 25 +
-                                128) >> 8) + 16);
-      yplane[x] = alpha_blend(alpha, color_y, yplane[x]);
-
-      // Only sample U and V at even coordinates.
-      if ((x % 2 == 0) && (y % 2 == 0)) {
-        int color_u = clip_byte(((color_r * -38 + color_g * -74 +
-                                  color_b * 112 + 128) >> 8) + 128);
-        int color_v = clip_byte(((color_r * 112 + color_g * -94 +
-                                  color_b * -18 + 128) >> 8) + 128);
-        uplane[x / 2] = alpha_blend(alpha, color_u, uplane[x / 2]);
-        vplane[x / 2] = alpha_blend(alpha, color_v, vplane[x / 2]);
-      }
-    }
-  }
-  cursor_bitmap.unlockPixels();
-}
-
-class DesktopVideoCaptureMachine
-    : public VideoCaptureMachine,
-      public aura::WindowObserver,
-      public ui::CompositorObserver,
-      public base::SupportsWeakPtr<DesktopVideoCaptureMachine> {
- public:
-  DesktopVideoCaptureMachine(const DesktopMediaID& source);
-  ~DesktopVideoCaptureMachine() override;
-
-  // VideoCaptureFrameSource overrides.
-  bool Start(const scoped_refptr<ThreadSafeCaptureOracle>& oracle_proxy,
-             const media::VideoCaptureParams& params) override;
-  void Stop(const base::Closure& callback) override;
-
-  // Implements aura::WindowObserver.
-  void OnWindowBoundsChanged(aura::Window* window,
-                             const gfx::Rect& old_bounds,
-                             const gfx::Rect& new_bounds) override;
-  void OnWindowDestroyed(aura::Window* window) override;
-  void OnWindowAddedToRootWindow(aura::Window* window) override;
-  void OnWindowRemovingFromRootWindow(aura::Window* window,
-                                      aura::Window* new_root) override;
-
-  // Implements ui::CompositorObserver.
-  void OnCompositingDidCommit(ui::Compositor* compositor) override {}
-  void OnCompositingStarted(ui::Compositor* compositor,
-                            base::TimeTicks start_time) override {}
-  void OnCompositingEnded(ui::Compositor* compositor) override;
-  void OnCompositingAborted(ui::Compositor* compositor) override {}
-  void OnCompositingLockStateChanged(ui::Compositor* compositor) override {}
-  void OnCompositingShuttingDown(ui::Compositor* compositor) override {}
-
- private:
-  // Captures a frame.
-  // |dirty| is false for timer polls and true for compositor updates.
-  void Capture(bool dirty);
-
-  // Update capture size. Must be called on the UI thread.
-  void UpdateCaptureSize();
-
-  // Response callback for cc::Layer::RequestCopyOfOutput().
-  void DidCopyOutput(
-      scoped_refptr<media::VideoFrame> video_frame,
-      base::TimeTicks start_time,
-      const ThreadSafeCaptureOracle::CaptureFrameCallback& capture_frame_cb,
-      scoped_ptr<cc::CopyOutputResult> result);
-
-  // A helper which does the real work for DidCopyOutput. Returns true if
-  // succeeded.
-  bool ProcessCopyOutputResponse(
-      scoped_refptr<media::VideoFrame> video_frame,
-      base::TimeTicks start_time,
-      const ThreadSafeCaptureOracle::CaptureFrameCallback& capture_frame_cb,
-      scoped_ptr<cc::CopyOutputResult> result);
-
-  // Helper function to update cursor state.
-  // |region_in_frame| defines the desktop bound in the captured frame.
-  // Returns the current cursor position in captured frame.
-  gfx::Point UpdateCursorState(const gfx::Rect& region_in_frame);
-
-  // Clears cursor state.
-  void ClearCursorState();
-
-  // The window associated with the desktop.
-  aura::Window* desktop_window_;
-
-  // The timer that kicks off period captures.
-  base::Timer timer_;
-
-  // The id of the window being captured.
-  DesktopMediaID window_id_;
-
-  // Makes all the decisions about which frames to copy, and how.
-  scoped_refptr<ThreadSafeCaptureOracle> oracle_proxy_;
-
-  // The capture parameters for this capture.
-  media::VideoCaptureParams capture_params_;
-
-  // YUV readback pipeline.
-  scoped_ptr<content::ReadbackYUVInterface> yuv_readback_pipeline_;
-
-  // Cursor state.
-  ui::Cursor last_cursor_;
-  gfx::Point cursor_hot_point_;
-  SkBitmap scaled_cursor_bitmap_;
-
-  // TODO(jiayl): Remove power_save_blocker_ when there is an API to keep the
-  // screen from sleeping for the drive-by web.
-  scoped_ptr<PowerSaveBlocker> power_save_blocker_;
-
-  DISALLOW_COPY_AND_ASSIGN(DesktopVideoCaptureMachine);
-};
-
-DesktopVideoCaptureMachine::DesktopVideoCaptureMachine(
-    const DesktopMediaID& source)
-    : desktop_window_(NULL),
-      timer_(true, true),
-      window_id_(source) {}
-
-DesktopVideoCaptureMachine::~DesktopVideoCaptureMachine() {}
-
-bool DesktopVideoCaptureMachine::Start(
-    const scoped_refptr<ThreadSafeCaptureOracle>& oracle_proxy,
-    const media::VideoCaptureParams& params) {
-  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
-
-  desktop_window_ = content::DesktopMediaID::GetAuraWindowById(window_id_);
-  if (!desktop_window_)
-    return false;
-
-  // If the associated layer is already destroyed then return failure.
-  ui::Layer* layer = desktop_window_->layer();
-  if (!layer)
-    return false;
-
-  DCHECK(oracle_proxy.get());
-  oracle_proxy_ = oracle_proxy;
-  capture_params_ = params;
-
-  // Update capture size.
-  UpdateCaptureSize();
-
-  // Start observing window events.
-  desktop_window_->AddObserver(this);
-
-  // Start observing compositor updates.
-  if (desktop_window_->GetHost())
-    desktop_window_->GetHost()->compositor()->AddObserver(this);
-
-  power_save_blocker_.reset(
-      PowerSaveBlocker::Create(
-          PowerSaveBlocker::kPowerSaveBlockPreventDisplaySleep,
-          PowerSaveBlocker::kReasonOther,
-          "DesktopCaptureDevice is running").release());
-
-  // Starts timer.
-  timer_.Start(FROM_HERE, oracle_proxy_->min_capture_period(),
-               base::Bind(&DesktopVideoCaptureMachine::Capture, AsWeakPtr(),
-                          false));
-
-  return true;
-}
-
-void DesktopVideoCaptureMachine::Stop(const base::Closure& callback) {
-  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
-  power_save_blocker_.reset();
-
-  // Stop observing compositor and window events.
-  if (desktop_window_) {
-    aura::WindowTreeHost* window_host = desktop_window_->GetHost();
-    // In the host destructor the compositor is destroyed before the window.
-    if (window_host && window_host->compositor())
-      window_host->compositor()->RemoveObserver(this);
-    desktop_window_->RemoveObserver(this);
-    desktop_window_ = NULL;
-  }
-
-  // Stop timer.
-  timer_.Stop();
-
-  callback.Run();
-}
-
-void DesktopVideoCaptureMachine::UpdateCaptureSize() {
-  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
-  if (oracle_proxy_.get() && desktop_window_) {
-     ui::Layer* layer = desktop_window_->layer();
-     oracle_proxy_->UpdateCaptureSize(ui::ConvertSizeToPixel(
-         layer, layer->bounds().size()));
-  }
-  ClearCursorState();
-}
-
-void DesktopVideoCaptureMachine::Capture(bool dirty) {
-  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
-
-  // Do not capture if the desktop window is already destroyed.
-  if (!desktop_window_)
-    return;
-
-  scoped_refptr<media::VideoFrame> frame;
-  ThreadSafeCaptureOracle::CaptureFrameCallback capture_frame_cb;
-
-  const base::TimeTicks start_time = base::TimeTicks::Now();
-  const VideoCaptureOracle::Event event =
-      dirty ? VideoCaptureOracle::kCompositorUpdate
-            : VideoCaptureOracle::kTimerPoll;
-  if (oracle_proxy_->ObserveEventAndDecideCapture(
-          event, gfx::Rect(), start_time, &frame, &capture_frame_cb)) {
-    scoped_ptr<cc::CopyOutputRequest> request =
-        cc::CopyOutputRequest::CreateRequest(
-            base::Bind(&DesktopVideoCaptureMachine::DidCopyOutput,
-                       AsWeakPtr(), frame, start_time, capture_frame_cb));
-    gfx::Rect window_rect = gfx::Rect(desktop_window_->bounds().width(),
-                                      desktop_window_->bounds().height());
-    request->set_area(window_rect);
-    desktop_window_->layer()->RequestCopyOfOutput(request.Pass());
-  }
-}
-
-void CopyOutputFinishedForVideo(
-    base::TimeTicks start_time,
-    const ThreadSafeCaptureOracle::CaptureFrameCallback& capture_frame_cb,
-    const scoped_refptr<media::VideoFrame>& target,
-    const SkBitmap& cursor_bitmap,
-    const gfx::Point& cursor_position,
-    scoped_ptr<cc::SingleReleaseCallback> release_callback,
-    bool result) {
-  if (!cursor_bitmap.isNull())
-    RenderCursorOnVideoFrame(target, cursor_bitmap, cursor_position);
-  release_callback->Run(0, false);
-  capture_frame_cb.Run(target, start_time, result);
-}
-
-void RunSingleReleaseCallback(scoped_ptr<cc::SingleReleaseCallback> cb,
-                              uint32 sync_point) {
-  cb->Run(sync_point, false);
-}
-
-void DesktopVideoCaptureMachine::DidCopyOutput(
-    scoped_refptr<media::VideoFrame> video_frame,
-    base::TimeTicks start_time,
-    const ThreadSafeCaptureOracle::CaptureFrameCallback& capture_frame_cb,
-    scoped_ptr<cc::CopyOutputResult> result) {
-  static bool first_call = true;
-
-  bool succeeded = ProcessCopyOutputResponse(
-      video_frame, start_time, capture_frame_cb, result.Pass());
-
-  base::TimeDelta capture_time = base::TimeTicks::Now() - start_time;
-
-  // The two UMA_ blocks must be put in its own scope since it creates a static
-  // variable which expected constant histogram name.
-  if (window_id_.type == DesktopMediaID::TYPE_SCREEN) {
-    UMA_HISTOGRAM_TIMES(kUmaScreenCaptureTime, capture_time);
-  } else {
-    UMA_HISTOGRAM_TIMES(kUmaWindowCaptureTime, capture_time);
-  }
-
-  if (first_call) {
-    first_call = false;
-    if (window_id_.type == DesktopMediaID::TYPE_SCREEN) {
-      IncrementDesktopCaptureCounter(succeeded ? FIRST_SCREEN_CAPTURE_SUCCEEDED
-                                               : FIRST_SCREEN_CAPTURE_FAILED);
-    } else {
-      IncrementDesktopCaptureCounter(succeeded
-                                         ? FIRST_WINDOW_CAPTURE_SUCCEEDED
-                                         : FIRST_WINDOW_CAPTURE_FAILED);
-    }
-  }
-}
-
-bool DesktopVideoCaptureMachine::ProcessCopyOutputResponse(
-    scoped_refptr<media::VideoFrame> video_frame,
-    base::TimeTicks start_time,
-    const ThreadSafeCaptureOracle::CaptureFrameCallback& capture_frame_cb,
-    scoped_ptr<cc::CopyOutputResult> result) {
-  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
-  if (result->IsEmpty() || result->size().IsEmpty() || !desktop_window_)
-    return false;
-
-  if (capture_params_.requested_format.pixel_format ==
-      media::PIXEL_FORMAT_TEXTURE) {
-    DCHECK(!video_frame.get());
-    cc::TextureMailbox texture_mailbox;
-    scoped_ptr<cc::SingleReleaseCallback> release_callback;
-    result->TakeTexture(&texture_mailbox, &release_callback);
-    DCHECK(texture_mailbox.IsTexture());
-    if (!texture_mailbox.IsTexture())
-      return false;
-    video_frame = media::VideoFrame::WrapNativeTexture(
-        make_scoped_ptr(new gpu::MailboxHolder(texture_mailbox.mailbox(),
-                                               texture_mailbox.target(),
-                                               texture_mailbox.sync_point())),
-        base::Bind(&RunSingleReleaseCallback, base::Passed(&release_callback)),
-        result->size(), gfx::Rect(result->size()), result->size(),
-        base::TimeDelta(), false);
-    capture_frame_cb.Run(video_frame, start_time, true);
-    return true;
-  }
-
-  // Compute the dest size we want after the letterboxing resize. Make the
-  // coordinates and sizes even because we letterbox in YUV space
-  // (see CopyRGBToVideoFrame). They need to be even for the UV samples to
-  // line up correctly.
-  // The video frame's visible_rect() and the result's size() are both physical
-  // pixels.
-  gfx::Rect region_in_frame = media::ComputeLetterboxRegion(
-      video_frame->visible_rect(), result->size());
-  region_in_frame = gfx::Rect(region_in_frame.x() & ~1,
-                              region_in_frame.y() & ~1,
-                              region_in_frame.width() & ~1,
-                              region_in_frame.height() & ~1);
-  if (region_in_frame.IsEmpty())
-    return false;
-
-  ImageTransportFactory* factory = ImageTransportFactory::GetInstance();
-  GLHelper* gl_helper = factory->GetGLHelper();
-  if (!gl_helper)
-    return false;
-
-  cc::TextureMailbox texture_mailbox;
-  scoped_ptr<cc::SingleReleaseCallback> release_callback;
-  result->TakeTexture(&texture_mailbox, &release_callback);
-  DCHECK(texture_mailbox.IsTexture());
-  if (!texture_mailbox.IsTexture())
-    return false;
-
-  gfx::Rect result_rect(result->size());
-  if (!yuv_readback_pipeline_ ||
-      yuv_readback_pipeline_->scaler()->SrcSize() != result_rect.size() ||
-      yuv_readback_pipeline_->scaler()->SrcSubrect() != result_rect ||
-      yuv_readback_pipeline_->scaler()->DstSize() != region_in_frame.size()) {
-    yuv_readback_pipeline_.reset(
-        gl_helper->CreateReadbackPipelineYUV(GLHelper::SCALER_QUALITY_FAST,
-                                             result_rect.size(),
-                                             result_rect,
-                                             region_in_frame.size(),
-                                             true,
-                                             true));
-  }
-
-  gfx::Point cursor_position_in_frame = UpdateCursorState(region_in_frame);
-  yuv_readback_pipeline_->ReadbackYUV(
-      texture_mailbox.mailbox(),
-      texture_mailbox.sync_point(),
-      video_frame.get(),
-      region_in_frame.origin(),
-      base::Bind(&CopyOutputFinishedForVideo,
-                 start_time,
-                 capture_frame_cb,
-                 video_frame,
-                 scaled_cursor_bitmap_,
-                 cursor_position_in_frame,
-                 base::Passed(&release_callback)));
-  return true;
-}
-
-gfx::Point DesktopVideoCaptureMachine::UpdateCursorState(
-    const gfx::Rect& region_in_frame) {
-  const gfx::Rect desktop_bounds = desktop_window_->layer()->bounds();
-  gfx::NativeCursor cursor =
-      desktop_window_->GetHost()->last_cursor();
-  if (last_cursor_ != cursor) {
-    SkBitmap cursor_bitmap;
-    if (ui::GetCursorBitmap(cursor, &cursor_bitmap, &cursor_hot_point_)) {
-      scaled_cursor_bitmap_ = skia::ImageOperations::Resize(
-          cursor_bitmap,
-          skia::ImageOperations::RESIZE_BEST,
-          cursor_bitmap.width() * region_in_frame.width() /
-              desktop_bounds.width(),
-          cursor_bitmap.height() * region_in_frame.height() /
-              desktop_bounds.height());
-      last_cursor_ = cursor;
-    } else {
-      // Clear cursor state if ui::GetCursorBitmap failed so that we do not
-      // render cursor on the captured frame.
-      ClearCursorState();
-    }
-  }
-
-  gfx::Point cursor_position = aura::Env::GetInstance()->last_mouse_location();
-  aura::client::GetScreenPositionClient(desktop_window_->GetRootWindow())->
-      ConvertPointFromScreen(desktop_window_, &cursor_position);
-  const gfx::Point hot_point_in_dip = ui::ConvertPointToDIP(
-      desktop_window_->layer(), cursor_hot_point_);
-  cursor_position.Offset(-desktop_bounds.x() - hot_point_in_dip.x(),
-                         -desktop_bounds.y() - hot_point_in_dip.y());
-  return gfx::Point(
-      region_in_frame.x() + cursor_position.x() * region_in_frame.width() /
-          desktop_bounds.width(),
-      region_in_frame.y() + cursor_position.y() * region_in_frame.height() /
-          desktop_bounds.height());
-}
-
-void DesktopVideoCaptureMachine::ClearCursorState() {
-  last_cursor_ = ui::Cursor();
-  cursor_hot_point_ = gfx::Point();
-  scaled_cursor_bitmap_.reset();
-}
-
-void DesktopVideoCaptureMachine::OnWindowBoundsChanged(
-    aura::Window* window,
-    const gfx::Rect& old_bounds,
-    const gfx::Rect& new_bounds) {
-  DCHECK(desktop_window_ && window == desktop_window_);
-
-  // Post task to update capture size on UI thread.
-  BrowserThread::PostTask(BrowserThread::UI, FROM_HERE, base::Bind(
-      &DesktopVideoCaptureMachine::UpdateCaptureSize, AsWeakPtr()));
-}
-
-void DesktopVideoCaptureMachine::OnWindowDestroyed(aura::Window* window) {
-  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
-
-  Stop(base::Bind(&base::DoNothing));
-
-  oracle_proxy_->ReportError("OnWindowDestroyed()");
-}
-
-void DesktopVideoCaptureMachine::OnWindowAddedToRootWindow(
-    aura::Window* window) {
-  DCHECK(window == desktop_window_);
-  window->GetHost()->compositor()->AddObserver(this);
-}
-
-void DesktopVideoCaptureMachine::OnWindowRemovingFromRootWindow(
-    aura::Window* window,
-    aura::Window* new_root) {
-  DCHECK(window == desktop_window_);
-  window->GetHost()->compositor()->RemoveObserver(this);
-}
-
-void DesktopVideoCaptureMachine::OnCompositingEnded(
-    ui::Compositor* compositor) {
-  BrowserThread::PostTask(BrowserThread::UI, FROM_HERE, base::Bind(
-      &DesktopVideoCaptureMachine::Capture, AsWeakPtr(), true));
-}
-
-}  // namespace
-
-DesktopCaptureDeviceAura::DesktopCaptureDeviceAura(
-    const DesktopMediaID& source)
-    : core_(new ContentVideoCaptureDeviceCore(scoped_ptr<VideoCaptureMachine>(
-        new DesktopVideoCaptureMachine(source)))) {}
-
 DesktopCaptureDeviceAura::~DesktopCaptureDeviceAura() {
   DVLOG(2) << "DesktopCaptureDeviceAura@" << this << " destroying.";
 }
 
 // static
 media::VideoCaptureDevice* DesktopCaptureDeviceAura::Create(
     const DesktopMediaID& source) {
-  IncrementDesktopCaptureCounter(source.type == DesktopMediaID::TYPE_SCREEN
-                                     ? SCREEN_CAPTURER_CREATED
-                                     : WINDOW_CAPTURER_CREATED);
-  return new DesktopCaptureDeviceAura(source);
+  DesktopCaptureDeviceAura* device = new DesktopCaptureDeviceAura();
+
+  BrowserThread::PostTask(BrowserThread::UI, FROM_HERE,
+                          base::Bind(&DesktopCaptureDeviceAura::SetWindow,
+                                     device->AsWeakPtr(), source));

[Sergey Ulanov, 2015/03/04 01:54:03]

weak pointers are not thread-safe and shouldn't be used to post tasks from one
thread to another. I believe this code should DCHECK in debug builds. See my
comment below for how you can avoid weak pointers here.

+  return device;
 }
 
 void DesktopCaptureDeviceAura::AllocateAndStart(
     const media::VideoCaptureParams& params,
     scoped_ptr<Client> client) {
   DVLOG(1) << "Allocating " << params.requested_format.frame_size.ToString();
   core_->AllocateAndStart(params, client.Pass());
 }
 
 void DesktopCaptureDeviceAura::StopAndDeAllocate() {
   core_->StopAndDeAllocate();
 }
 
+void DesktopCaptureDeviceAura::SetWindow(const DesktopMediaID& source) {

[Sergey Ulanov, 2015/03/04 01:54:03]

This doesn't need to be a class member. You can have a static function instead
that takes AuraWindowCaptureMachine* and DesktopMediaID as arguments, resolves
DesktopMediaID and passes it to the AuraWindowCaptureMachine::SetWindow().
Passing raw pointer to AuraWindowCaptureMachine is safe because it's always
destroyed on the UI thread. That way you can avoid the problem mentioned above.

[robert.bradford, 2015/03/04 18:33:05]

Ah, this was the insight that I was missing and I also wrongly thought that
WeakPtr was the solution. The existing code makes use of them when posting tasks
however I missed that the request is made from the UI thread so the WeakPtr is
safe.

+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  aura::Window* window = DesktopMediaID::GetAuraWindowById(source);
+  machine_->SetWindow(window);
+}
+
 }  // namespace content