Fix stale compositor references from ui::Layer

ui/gfx/compositor/compositor.cc

Index: ui/gfx/compositor/compositor.cc
diff --git a/ui/gfx/compositor/compositor.cc b/ui/gfx/compositor/compositor.cc
index 6e9e1d5e216b3d92af98142906cbcfe7fe3af7a6..485dc2d5c6468ff0ab368a7e944ea79b0a292784 100644
--- a/ui/gfx/compositor/compositor.cc
+++ b/ui/gfx/compositor/compositor.cc
@@ -26,6 +26,8 @@ Compositor::Compositor(CompositorDelegate* delegate, const gfx::Size& size)
 }
 
 Compositor::~Compositor() {
+  if (root_layer_)
+    root_layer_->SetCompositor(NULL);
 }
 
 void Compositor::ScheduleDraw() {
@@ -33,8 +35,10 @@ void Compositor::ScheduleDraw() {
 }
 
 void Compositor::SetRootLayer(Layer* root_layer) {
+  if (root_layer_)
+    root_layer_->SetCompositor(NULL);
   root_layer_ = root_layer;
-  if (!root_layer_->GetCompositor())
+  if (root_layer_ && !root_layer_->GetCompositor())
     root_layer_->SetCompositor(this);
   OnRootLayerChanged();
 }