Makes DefaultCaptureClient work with a null root_window

ui/aura/client/default_capture_client.cc

 // 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.
 
 #include "ui/aura/client/default_capture_client.h"
 
 #include "ui/aura/client/capture_client_observer.h"
 #include "ui/aura/window.h"
 #include "ui/aura/window_event_dispatcher.h"
 #include "ui/aura/window_tree_host.h"
 
 namespace aura {
 namespace client {
 namespace {
 
 // Track the active capture window across root windows.
 Window* global_capture_window_ = nullptr;
 
 }  // namespace
 
 DefaultCaptureClient::DefaultCaptureClient(Window* root_window)
     : root_window_(root_window), capture_window_(nullptr) {
-  SetCaptureClient(root_window_, this);
+  if (root_window_)
+    SetCaptureClient(root_window_, this);
 }
 
 DefaultCaptureClient::~DefaultCaptureClient() {
   if (global_capture_window_ == capture_window_)
     global_capture_window_ = nullptr;
-  SetCaptureClient(root_window_, nullptr);
+  if (root_window_)
+    SetCaptureClient(root_window_, nullptr);
 }
 
 void DefaultCaptureClient::SetCapture(Window* window) {
   if (capture_window_ == window)
     return;
   if (window)
     ui::GestureRecognizer::Get()->CancelActiveTouchesExcept(window);
 
   Window* old_capture_window = capture_window_;
   capture_window_ = window;
   global_capture_window_ = window;
 
-  CaptureDelegate* capture_delegate = root_window_->GetHost()->dispatcher();
-  if (capture_window_)
+  CaptureDelegate* capture_delegate = nullptr;
+  if (capture_window_) {
+    capture_delegate = capture_window_->GetHost()->dispatcher();

[msw, 2016/10/25 17:57:58]

q: just checking that the change from |root_window_| to |capture_window_| and
|old_capture_window_| to get the WindowTreeHost and WindowEventDispatcher is
intentional; should we check that the root of the [old] capture window matches
|root_window_| if that value is non-null?

     capture_delegate->SetNativeCapture();
-  else
+  } else {
+    capture_delegate = old_capture_window->GetHost()->dispatcher();
     capture_delegate->ReleaseNativeCapture();
+  }
 
   capture_delegate->UpdateCapture(old_capture_window, capture_window_);
 
   for (CaptureClientObserver& observer : observers_)
     observer.OnCaptureChanged(old_capture_window, capture_window_);
 }
 
 void DefaultCaptureClient::ReleaseCapture(Window* window) {
   if (capture_window_ != window)
     return;
@@ skipping 11 matching lines @@
 void DefaultCaptureClient::AddObserver(CaptureClientObserver* observer) {
   observers_.AddObserver(observer);
 }
 
 void DefaultCaptureClient::RemoveObserver(CaptureClientObserver* observer) {
   observers_.RemoveObserver(observer);
 }
 
 }  // namespace client
 }  // namespace aura