Fix stale compositor references from ui::Layer

ui/gfx/compositor/compositor.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_GFX_COMPOSITOR_COMPOSITOR_H_
 #define UI_GFX_COMPOSITOR_COMPOSITOR_H_
 #pragma once
 
 #include "base/memory/ref_counted.h"
 #include "base/observer_list.h"
@@ skipping 104 matching lines @@
 
   // Creates a new texture. The caller owns the returned object.
   virtual Texture* CreateTexture() = 0;
 
   // Blurs the specific region in the compositor.
   virtual void Blur(const gfx::Rect& bounds) = 0;
 
   // Schedules a redraw of the layer tree associated with this compositor.
   virtual void ScheduleDraw();
 
-  // Sets the root of the layer tree drawn by this Compositor.
+  // Sets the root of the layer tree drawn by this Compositor. The root layer
+  // must have no parent. The compositor's root layer is reset if the root layer
+  // is destroyed. NULL can be passed to reset the root layer, in which case the
+  // compositor will stop drawing anything.
   // The Compositor does not own the root layer.
   const Layer* root_layer() const { return root_layer_; }
   Layer* root_layer() { return root_layer_; }
   void SetRootLayer(Layer* root_layer);
 
   // Draws the scene created by the layer tree and any visual effects. If
   // |force_clear| is true, this will cause the compositor to clear before
   // compositing.
   void Draw(bool force_clear);
 
@@ skipping 71 matching lines @@
   // calls to it.
   static ui::Compositor*(*compositor_factory_)(
       ui::CompositorDelegate* delegate);
 
   friend class base::RefCounted<Compositor>;
 };
 
 }  // namespace ui
 
 #endif  // UI_GFX_COMPOSITOR_COMPOSITOR_H_