cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/output/output_surface_frame.h

Index: cc/output/output_surface_frame.h
diff --git a/cc/output/output_surface_frame.h b/cc/output/output_surface_frame.h
index 2306f2e5b4bbe686807d324dd438eee65266d245..a221686dfcdc2b3c89d63fb9dd763883abaad0b5 100644
--- a/cc/output/output_surface_frame.h
+++ b/cc/output/output_surface_frame.h
@@ -29,6 +29,11 @@ class CC_EXPORT OutputSurfaceFrame {
   gfx::Rect sub_buffer_rect;
   std::vector<ui::LatencyInfo> latency_info;
 
+  // See corresponding fields in CompositorFrameMetadata.
+  uint64_t begin_frame_source_id = 0;
+  uint64_t begin_frame_number = 0;
+  uint64_t latest_confirmed_frame = 0;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(OutputSurfaceFrame);
 };