Send FrameSwapMessageQueue's messages with a separate IPC

cc/ipc/compositor_frame_metadata.mojom

Index: cc/ipc/compositor_frame_metadata.mojom
diff --git a/cc/ipc/compositor_frame_metadata.mojom b/cc/ipc/compositor_frame_metadata.mojom
index 45d67c763a29fca00b7ffe562658df34b6c88b4d..eafa877100ea369e05aeeea58fe3d46024f67246 100644
--- a/cc/ipc/compositor_frame_metadata.mojom
+++ b/cc/ipc/compositor_frame_metadata.mojom
@@ -34,4 +34,5 @@ struct CompositorFrameMetadata {
   bool can_activate_before_dependencies;
   uint32 content_source_id;
   BeginFrameAck begin_frame_ack;
+  uint32 frame_token;

[Tom Sepez, 2017/04/04 18:11:04]

Does it matter that these are guessable?
It looks like we shoot the renderer if it sends a bad token, which is a good
thing, but can competing renderers muck with each other's tokens?

[Saman Sami, 2017/04/04 18:25:24]

Since a RenderWidget cannot send messages to RenderWidgetHostImpl belonging to
another process, I don't believe one renderer can cause problems for another
one. The frame token is basically a way to make distinction between different
frames sent by RenderWidget so in the browser process we can say this set of
messages was meant for frame 1 and these set of messages belong to frame 2.
Another renderer cannot send messages on behalf of another renderer or execute
messages prematurely because it doesn't have access to RenderHidgetHostImpls for
another renderer.

 };