Fix stale compositor references from ui::Layer

ui/gfx/compositor/compositor.h

Index: ui/gfx/compositor/compositor.h
diff --git a/ui/gfx/compositor/compositor.h b/ui/gfx/compositor/compositor.h
index 33a7bcee265c1010787ef8b003723920e86ed447..23fff5b728948039f1a8ec011190cef0061fc702 100644
--- a/ui/gfx/compositor/compositor.h
+++ b/ui/gfx/compositor/compositor.h
@@ -122,7 +122,10 @@ class COMPOSITOR_EXPORT Compositor : public base::RefCounted<Compositor> {
   // 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_; }