Fixes some asserts in compositor_unittests

ui/gfx/compositor/compositor.cc

 // 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.
 
 #include "ui/gfx/compositor/compositor.h"
 
 #include "ui/gfx/compositor/compositor_observer.h"
 #include "ui/gfx/compositor/layer.h"
 
 namespace ui {
@@ skipping 17 matching lines @@
 Compositor::~Compositor() {
   if (root_layer_)
     root_layer_->SetCompositor(NULL);
 }
 
 void Compositor::ScheduleDraw() {
   delegate_->ScheduleDraw();
 }
 
 void Compositor::SetRootLayer(Layer* root_layer) {
+  if (root_layer_ == root_layer)
+    return;
   if (root_layer_)
     root_layer_->SetCompositor(NULL);
   root_layer_ = root_layer;
   if (root_layer_ && !root_layer_->GetCompositor())
     root_layer_->SetCompositor(this);
   OnRootLayerChanged();
 }
 
 void Compositor::Draw(bool force_clear) {
   if (!root_layer_)
@@ skipping 52 matching lines @@
 }
 
 void Compositor::NotifyEnd() {
   OnNotifyEnd();
   FOR_EACH_OBSERVER(CompositorObserver,
                     observer_list_,
                     OnCompositingEnded(this));
 }
 
 }  // namespace ui