cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/ipc/begin_frame_args.mojom

Index: cc/ipc/begin_frame_args.mojom
diff --git a/cc/ipc/begin_frame_args.mojom b/cc/ipc/begin_frame_args.mojom
index b0b784a445ec35ec5b93117b6234c220c126709b..9c13227c467c52c9dd487fd31f755d837252400c 100644
--- a/cc/ipc/begin_frame_args.mojom
+++ b/cc/ipc/begin_frame_args.mojom
@@ -15,9 +15,20 @@ enum BeginFrameArgsType {
 
 // See cc/output/begin_frame_args.h.
 struct BeginFrameArgs {
+  uint64 source_id;
+  uint64 sequence_number;
   mojo.common.mojom.TimeTicks frame_time;
   mojo.common.mojom.TimeTicks deadline;
   mojo.common.mojom.TimeDelta interval;
   BeginFrameArgsType type;
   bool on_critical_path;
 };
+
+// See cc/output/begin_frame_args.h.
+struct BeginFrameAck {
+  uint64 source_id;
+  uint64 sequence_number;
+  bool has_updates;
+  uint64 oldest_incorporated_frame;
+  uint32 remaining_frames;
+};