cc: Introduce BeginFrame sequence numbers and acknowledgements.

content/renderer/android/synchronous_compositor_frame_sink.cc

Index: content/renderer/android/synchronous_compositor_frame_sink.cc
diff --git a/content/renderer/android/synchronous_compositor_frame_sink.cc b/content/renderer/android/synchronous_compositor_frame_sink.cc
index 228b5698bddf309c2e57f0407604508b54a29ad5..451efce291a0133c0aec0dede08f1c67ba7f07bd 100644
--- a/content/renderer/android/synchronous_compositor_frame_sink.cc
+++ b/content/renderer/android/synchronous_compositor_frame_sink.cc
@@ -471,8 +471,8 @@ void SynchronousCompositorFrameSink::ReturnResources(
 
 void SynchronousCompositorFrameSink::SetBeginFrameSource(
     cc::BeginFrameSource* begin_frame_source) {
-  // Software output is synchronous and doesn't use a BeginFrameSource.
-  NOTREACHED();
+  // Software output is synchronous and doesn't use the frame sink's (Display's)
+  // BeginFrameSource.
 }
 
 }  // namespace content