Crash fix for AuraWindowCaptureMachine/ReadbackYUV

Adds missing ContextFactoryObserver::OnLostResources() implementation
that is required for when GLHelper is destroyed/re-created because the
current ReadbackYUVPipeline instance is no longer valid.

TBR=wez@chromium.org

Bug: 760251
Change-Id: I85f54b60b32501aa3e08e1dcca7891ce739a913a
Reviewed-on: https://chromium-review.googlesource.com/647409
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#499354}

content/browser/media/capture/aura_window_capture_machine.cc

@@ -81,6 +81,9 @@ bool AuraWindowCaptureMachine::InternalStart(
   // Update capture size.
   UpdateCaptureSize();
 
+  // Start observing for GL context losses.
+  ImageTransportFactory::GetInstance()->GetContextFactory()->AddObserver(this);
+
   // Start observing compositor updates.
   aura::WindowTreeHost* const host = desktop_window_->GetHost();
   ui::Compositor* const compositor = host ? host->compositor() : nullptr;
@@ -165,6 +168,11 @@ void AuraWindowCaptureMachine::InternalStop(const base::Closure& callback) {
     cursor_renderer_.reset();
   }
 
+  // Stop observing for GL context losses.
+  ImageTransportFactory::GetInstance()->GetContextFactory()->RemoveObserver(
+      this);
+  OnLostResources();
+
   callback.Run();
 }
 
@@ -462,4 +470,9 @@ void AuraWindowCaptureMachine::OnCompositingShuttingDown(
   compositor->RemoveAnimationObserver(this);
 }
 
+void AuraWindowCaptureMachine::OnLostResources() {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  yuv_readback_pipeline_.reset();
+}
+
 }  // namespace content

content/browser/media/capture/aura_window_capture_machine.h

@@ -25,10 +25,10 @@ class ReadbackYUVInterface;
 
 namespace content {
 
-class AuraWindowCaptureMachine
-    : public media::VideoCaptureMachine,
-      public aura::WindowObserver,
-      public ui::CompositorAnimationObserver {
+class AuraWindowCaptureMachine : public media::VideoCaptureMachine,
+                                 public aura::WindowObserver,
+                                 public ui::ContextFactoryObserver,
+                                 public ui::CompositorAnimationObserver {
  public:
   AuraWindowCaptureMachine();
   ~AuraWindowCaptureMachine() override;
@@ -92,6 +92,9 @@ class AuraWindowCaptureMachine
                                  const CaptureFrameCallback& capture_frame_cb,
                                  std::unique_ptr<viz::CopyOutputResult> result);
 
+  // ui::ContextFactoryObserver implementation.
+  void OnLostResources() override;
+
   // Renders the cursor if needed and then delivers the captured frame.
   static void CopyOutputFinishedForVideo(
       base::WeakPtr<AuraWindowCaptureMachine> machine,