WebContentsVideoCapture Cleanup: Gut-out dead code, and tighten BUILDs.

content/browser/media/capture/web_contents_video_capture_device.h

 // Copyright (c) 2012 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.
 
 #ifndef CONTENT_BROWSER_MEDIA_CAPTURE_WEB_CONTENTS_VIDEO_CAPTURE_DEVICE_H_
 #define CONTENT_BROWSER_MEDIA_CAPTURE_WEB_CONTENTS_VIDEO_CAPTURE_DEVICE_H_
 
 #include <memory>
 #include <string>
 
 #include "base/macros.h"
 #include "content/common/content_export.h"
 #include "media/capture/content/screen_capture_device_core.h"
 #include "media/capture/video/video_capture_device.h"
 
 namespace content {
 
-// A virtualized VideoCaptureDevice that mirrors the displayed contents of a
+// A virtualized VideoCaptureDevice that captures the displayed contents of a
 // WebContents (i.e., the composition of an entire render frame tree), producing
-// a stream of video frames.
+// a stream of video frames. As such, WebContentsVideoCaptureDevice is only
+// supported on platforms that use the Chromium compositor, have a
+// content::RenderWidgetHostView implementation that supports frame subscription
+// (via BeginFrameSubscription()), and can perform read-back into
+// media::VideoFrames (i.e.,
+// RenderWidgetHostViewBase::CopyFromCompositingSurfaceToVideoFrame() is
+// functional).
 //
 // An instance is created by providing a device_id.  The device_id contains
 // information necessary for finding a WebContents instance.  From then on,
 // WebContentsVideoCaptureDevice will capture from the RenderWidgetHost that
 // encompasses the currently active RenderFrameHost tree for that that
 // WebContents instance.  As the RenderFrameHost tree mutates (e.g., due to page
 // navigations, or crashes/reloads), capture will continue without interruption.
 class CONTENT_EXPORT WebContentsVideoCaptureDevice
     : public media::VideoCaptureDevice {
  public:
@@ skipping 21 matching lines @@
 
   const std::unique_ptr<media::ScreenCaptureDeviceCore> core_;
 
   DISALLOW_COPY_AND_ASSIGN(WebContentsVideoCaptureDevice);
 };
 
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_MEDIA_CAPTURE_WEB_CONTENTS_VIDEO_CAPTURE_DEVICE_H_