cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/output/compositor_frame_metadata.h

Index: cc/output/compositor_frame_metadata.h
diff --git a/cc/output/compositor_frame_metadata.h b/cc/output/compositor_frame_metadata.h
index fb41651635b39fb291b28d896a801185f659a919..106d965299252cf97670d621f1113489d440cc32 100644
--- a/cc/output/compositor_frame_metadata.h
+++ b/cc/output/compositor_frame_metadata.h
@@ -82,6 +82,13 @@ class CC_EXPORT CompositorFrameMetadata {
   // This is the set of Surfaces that are referenced by this frame.
   std::vector<SurfaceId> referenced_surfaces;
 
+  // Source identifier of the BeginFrame that this frame is in response to.
+  uint64_t begin_frame_source_id = 0;
+  // Sequence number of the BeginFrame that this frame is in response to.
+  uint64_t begin_frame_number = 0;
+  // Sequence number of the latest BeginFrame that is confirmed by this frame.
+  uint64_t latest_confirmed_frame = 0;
+
  private:
   CompositorFrameMetadata(const CompositorFrameMetadata& other);
   CompositorFrameMetadata operator=(const CompositorFrameMetadata&) = delete;