Don't let unittests access LTH from the 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 860 matching lines @@
   // Source frames start at 0, so this is invalid.
   return -1;
 }
 
 void LayerTreeTest::DestroyLayerTreeHost() {
   if (layer_tree_host_ && layer_tree_host_->root_layer())
     layer_tree_host_->root_layer()->SetLayerTreeHost(NULL);
   layer_tree_host_ = nullptr;
 }
 
+LayerTreeHost* LayerTreeTest::layer_tree_host() {
+  // We check for a null proxy here as we sometimes ask for the layer tree host
+  // when the proxy does not exist, often for checking settings after a test has
+  // completed. For example, LTHPixelResourceTest::RunPixelResourceTest. See
+  // elsewhere in this file for other examples.
+  DCHECK(!proxy() || proxy()->IsMainThread() || proxy()->IsMainThreadBlocked());
+  return layer_tree_host_.get();
+}
+
 }  // namespace cc