mus: Embedder can request and observe BeginFrame for embedded client.

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..0abac597f54f75b4d200ce6e74f5bdb18deee040 100644
--- a/services/ui/public/interfaces/window_tree.mojom
+++ b/services/ui/public/interfaces/window_tree.mojom
@@ -5,6 +5,7 @@
 module ui.mojom;
 
 import "cc/ipc/frame_sink_id.mojom";
+import "cc/ipc/frame_sink_manager.mojom";
 import "cc/ipc/local_surface_id.mojom";
 import "cc/ipc/surface_id.mojom";
 import "cc/ipc/surface_info.mojom";
@@ -111,6 +112,10 @@ interface WindowTree {
   SetWindowBounds(uint32 change_id, uint32 window_id, gfx.mojom.Rect bounds,
                   cc.mojom.LocalSurfaceId? local_surface_id);
 
+  // Requests to be notified of begin-frame messages for the client window
+  // embedded in |window_id|.
+  RequestBeginFrames(uint32 window_id, cc.mojom.FrameSinkObserver observer);

[sky, 2017/05/14 16:26:58]

The browser and renderer already have a more direct communication channel. Can't
they communicate additional information over that channel rather than modifying
the ui service? In other words, why can't the renderer tell the browser about
frames directly?

+
   // Sets the client area of the specified window. The client area is specified
   // by way of insets. Everything outside of the insets, and not in
   // |additional_client_areas| is considered non-client area.