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 562 matching lines @@
 
   ASSERT_FALSE(layer_tree_host_.get());
   client_.reset();
   if (timed_out_) {
     FAIL() << "Test timed out";
     return;
   }
   AfterTest();
 }
 
+LayerTreeHost* LayerTreeTest::layer_tree_host() {
+  DCHECK(proxy()->IsMainThread() || proxy()->IsMainThreadBlocked());
+  return layer_tree_host_.get();
+}
+
 }  // namespace cc