Don't let unittests access LTH from the impl thread.

cc/test/layer_tree_test.h

 // Copyright 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 CC_TEST_LAYER_TREE_TEST_H_
 #define CC_TEST_LAYER_TREE_TEST_H_
 
 #include "base/memory/ref_counted.h"
 #include "base/threading/thread.h"
 #include "cc/animation/animation_delegate.h"
@@ skipping 180 matching lines @@
   }
   base::SingleThreadTaskRunner* MainThreadTaskRunner() {
     return main_task_runner_.get();
   }
   Proxy* proxy() const {
     return layer_tree_host_ ? layer_tree_host_->proxy() : NULL;
   }
 
   bool TestEnded() const { return ended_; }
 
-  LayerTreeHost* layer_tree_host() { return layer_tree_host_.get(); }
+  LayerTreeHost* layer_tree_host();
   bool delegating_renderer() const { return delegating_renderer_; }
   FakeOutputSurface* output_surface() { return output_surface_; }
   int LastCommittedSourceFrameNumber(LayerTreeHostImpl* impl) const;
 
   void DestroyLayerTreeHost();
 
   // By default, output surface recreation is synchronous.
   void RequestNewOutputSurface() override;
   // Override this for pixel tests, where you need a real output surface.
   virtual scoped_ptr<OutputSurface> CreateOutputSurface();
@@ skipping 160 matching lines @@
   SINGLE_AND_MULTI_THREAD_DIRECT_RENDERER_TEST_F(TEST_FIXTURE_NAME); \
   SINGLE_AND_MULTI_THREAD_DELEGATING_RENDERER_TEST_F(TEST_FIXTURE_NAME)
 
 // Some tests want to control when notify ready for activation occurs,
 // but this is not supported in the single-threaded case.
 #define SINGLE_AND_MULTI_THREAD_BLOCKNOTIFY_TEST_F(TEST_FIXTURE_NAME) \
   SINGLE_THREAD_NOIMPL_TEST_F(TEST_FIXTURE_NAME);                     \
   MULTI_THREAD_TEST_F(TEST_FIXTURE_NAME)
 
 #endif  // CC_TEST_LAYER_TREE_TEST_H_