Mozart: Directly associate the renderer with the view tree.

mojo/services/gfx/composition/interfaces/renderers.mojom

Index: mojo/services/gfx/composition/interfaces/renderers.mojom
diff --git a/mojo/services/gfx/composition/interfaces/renderers.mojom b/mojo/services/gfx/composition/interfaces/renderers.mojom
index 3d9eb4700c5272008d2572830c19fec54f0a22dd..2defa9e9f9e29e44ba0cd706777b51d3ff8b3c7c 100644
--- a/mojo/services/gfx/composition/interfaces/renderers.mojom
+++ b/mojo/services/gfx/composition/interfaces/renderers.mojom
@@ -32,6 +32,9 @@ interface Renderer {
   SetRootScene(SceneToken scene_token, uint32 scene_version,
     mojo.Rect viewport);
 
+  // Dissociates the root scene from the renderer.
+  ResetRootScene();
+
   // Provides an interface which can be used to perform hit tests on the
   // contents of the renderer's scene graph.
   GetHitTester(HitTester& hit_tester);