[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 8bd04f00d18ce426d4ff098a969ba8556f520f8a..7411d20e900132685e8f46921b792cf42e4ed17e 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"
@@ -88,6 +89,9 @@ class CC_EXPORT CompositorFrameMetadata {
   // become available in all renderer processes. See https://crbug.com/695579.
   uint32_t content_source_id = 0;
 
+  // BeginFrameAck for the BeginFrame that this CompositorFrame answers.
+  BeginFrameAck begin_frame_ack;
+
  private:
   CompositorFrameMetadata(const CompositorFrameMetadata& other);
   CompositorFrameMetadata operator=(const CompositorFrameMetadata&) = delete;