ui: No more TestCompositor. Use NullDraw contexts in unit tests.

ui/compositor/compositor.cc

 // Copyright (c) 2012 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/compositor/compositor.h"
 
 #include <algorithm>
 #include <deque>
 
 #include "base/bind.h"
@@ skipping 62 matching lines @@
 namespace ui {
 
 // static
 ContextFactory* ContextFactory::GetInstance() {
   DCHECK(g_context_factory);
   return g_context_factory;
 }
 
 // static
 void ContextFactory::SetInstance(ContextFactory* instance) {
+  DCHECK_NE(!!g_context_factory, !!instance);
   g_context_factory = instance;
 }
 
 Texture::Texture(bool flipped, const gfx::Size& size, float device_scale_factor)
     : size_(size),
       flipped_(flipped),
       device_scale_factor_(device_scale_factor) {
 }
 
 Texture::~Texture() {
@@ skipping 480 matching lines @@
     // CompositorObservers to be notified before starting another
     // draw cycle.
     ScheduleDraw();
   }
   FOR_EACH_OBSERVER(CompositorObserver,
                     observer_list_,
                     OnCompositingEnded(this));
 }
 
 }  // namespace ui