Implement aura::WindowPortMus::CreateCompositorFrameSink()

services/ui/public/interfaces/window_tree.mojom

Index: services/ui/public/interfaces/window_tree.mojom
diff --git a/services/ui/public/interfaces/window_tree.mojom b/services/ui/public/interfaces/window_tree.mojom
index 47c812cc250796164ad1aaa2d114650cb42fa949..3aeb71429e6ae1307579134734c6250d52b1d317 100644
--- a/services/ui/public/interfaces/window_tree.mojom
+++ b/services/ui/public/interfaces/window_tree.mojom
@@ -315,6 +315,12 @@ interface WindowTree {
   // Called by the client to cancel any in progress drag drop operation. This
   // will result in a change completed for the underlying change.
   CancelDragDrop(uint32 window_id);
+
+  // Get the FrameSinkId for the given window_id.
+  // TODO(penghuang): Remove this function when the FrameSinkId allocation is
+  // moved to client. http://crbug.com/712302
+  [Sync]
+  GetFrameSinkId(uint32 window_id) => (cc.mojom.FrameSinkId? frame_sink_id);
 };
 
 // Changes to windows are not sent to the connection that originated the