[cc] Fix CompositorFrameSinkSupport BeginFrameAck interface.

cc/output/begin_frame_args.h

Index: cc/output/begin_frame_args.h
diff --git a/cc/output/begin_frame_args.h b/cc/output/begin_frame_args.h
index 7a6128994f723054e7c6c40e757475850dafbe35..41b156ad708d33692eed652b8c58a4466df3b4f7 100644
--- a/cc/output/begin_frame_args.h
+++ b/cc/output/begin_frame_args.h
@@ -125,6 +125,10 @@ struct CC_EXPORT BeginFrameAck {
                 uint32_t remaining_frames,
                 bool has_damage);
 
+  // Creates a BeginFrameAck for a manual BeginFrame. Used when clients produce
+  // a CompositorFrame without prior BeginFrame, e.g. for synchronous drawing.
+  static BeginFrameAck CreateManualAckWithDamage();
+
   // Sequence number of the BeginFrame that is acknowledged.
   uint64_t sequence_number;