Send FrameSwapMessageQueue's messages with a separate IPC

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 d4ad80add31ec6ecec908e18e82432cff16e1a55..8aa5b5141ce493b84e2e88aef9f1c29005ced5d4 100644
--- a/cc/output/compositor_frame_metadata.h
+++ b/cc/output/compositor_frame_metadata.h
@@ -92,6 +92,11 @@ class CC_EXPORT CompositorFrameMetadata {
   // BeginFrameAck for the BeginFrame that this CompositorFrame answers.
   BeginFrameAck begin_frame_ack;
 
+  // Once the display compositor processes a frame containing a non-zero frame
+  // token, the token is sent to embedder of the frame. This is helpful when
+  // the embedder wants to do something after a particular frame is processed.
+  uint32_t frame_token = 0;
+
  private:
   CompositorFrameMetadata(const CompositorFrameMetadata& other);
   CompositorFrameMetadata operator=(const CompositorFrameMetadata&) = delete;