[cc] Set BeginFrame sequence numbers on CompositorFrames from Scheduler.

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 e383e5069dcd82c0f9c4a044fd4f38d6eabc4969..16758d5431b3c854e94c1d5f9a842d9287c9d911 100644
--- a/cc/output/compositor_frame_metadata.h
+++ b/cc/output/compositor_frame_metadata.h
@@ -11,6 +11,7 @@
 
 #include "cc/base/cc_export.h"
 #include "cc/input/selection.h"
+#include "cc/output/begin_frame_args.h"
 #include "cc/surfaces/surface_id.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "ui/events/latency_info.h"
@@ -82,6 +83,9 @@ class CC_EXPORT CompositorFrameMetadata {
   // dependencies have been resolved.
   bool can_activate_before_dependencies = true;
 
+  // BeginFrameAck for the BeginFrame that this CompositorFrame answers.
+  BeginFrameAck begin_frame_ack;
+
  private:
   CompositorFrameMetadata(const CompositorFrameMetadata& other);
   CompositorFrameMetadata operator=(const CompositorFrameMetadata&) = delete;