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 c0ff4c0f9fbb6281aff35e0251950e5a3900084a..bb295514f4b651b1df77899c745cd17f19c29629 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