Reland "Shouldn't access LayerTreeTest::layer_tree_host on impl thread"

cc/test/layer_tree_test.cc

 // 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.
 
 #include "cc/test/layer_tree_test.h"
 
 #include "base/command_line.h"
 #include "cc/animation/animation.h"
 #include "cc/animation/animation_registrar.h"
 #include "cc/animation/layer_animation_controller.h"
@@ skipping 609 matching lines @@
 }
 
 scoped_refptr<cc::ContextProvider> LayerTreeTest::
     OffscreenContextProviderForCompositorThread() {
   if (!compositor_thread_contexts_.get() ||
       compositor_thread_contexts_->DestroyedOnMainThread())
     compositor_thread_contexts_ = FakeContextProvider::Create();
   return compositor_thread_contexts_;
 }
 
+LayerTreeHost* LayerTreeTest::layer_tree_host() {
+  DCHECK(proxy()->IsMainThread() || proxy()->IsMainThreadBlocked());
+  return layer_tree_host_.get();
+}
+
 }  // namespace cc