[cc] Add and use BeginFrameAck for DidFinishFrame.

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 142f7d0cdad3f56f5a52bd6e96ac8ea37af71801..89c1c670fe8d823c0d561d2ddd19795b1a4468cb 100644
--- a/cc/ipc/begin_frame_args.mojom
+++ b/cc/ipc/begin_frame_args.mojom
@@ -23,3 +23,12 @@ struct BeginFrameArgs {
   BeginFrameArgsType type;
   bool on_critical_path;
 };
+
+// See cc/output/begin_frame_args.h.
+struct BeginFrameAck {
+  uint64 source_id;
+  uint64 sequence_number;
+  uint64 latest_confirmed_frame;
+  uint32 remaining_frames;
+  bool has_damage;
+};