Ensure that mask layer is of the same size as the owning layer.

cc/layers/layer.cc

 // Copyright 2010 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/layers/layer.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 1189 matching lines @@
   layer->SetUpdateRect(inputs_.update_rect);
 
   layer->SetHasWillChangeTransformHint(has_will_change_transform_hint());
   layer->SetNeedsPushProperties();
 
   // Reset any state that should be cleared for the next update.
   needs_show_scrollbars_ = false;
   subtree_property_changed_ = false;
   inputs_.update_rect = gfx::Rect();
 
+  if (mask_layer())
+    DCHECK_EQ(bounds().ToString(), mask_layer()->bounds().ToString());
   layer_tree_host_->RemoveLayerShouldPushProperties(this);
 }
 
 void Layer::TakeCopyRequests(
     std::vector<std::unique_ptr<CopyOutputRequest>>* requests) {
   for (auto& it : inputs_.copy_requests) {
     scoped_refptr<base::SingleThreadTaskRunner> main_thread_task_runner =
         layer_tree_host()->GetTaskRunnerProvider()->MainThreadTaskRunner();
     std::unique_ptr<CopyOutputRequest> original_request = std::move(it);
     const CopyOutputRequest& original_request_ref = *original_request;
@@ skipping 208 matching lines @@
       ->subtree_has_copy_request;
 }
 
 gfx::Transform Layer::ScreenSpaceTransform() const {
   DCHECK_NE(transform_tree_index_, TransformTree::kInvalidNodeId);
   return draw_property_utils::ScreenSpaceTransform(
       this, layer_tree_host_->property_trees()->transform_tree);
 }
 
 }  // namespace cc