Ensure the viewport is always represented by the inner viewport layer.

cc/trees/layer_tree_host.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/trees/layer_tree_host.h"
 
 #include <algorithm>
 #include <stack>
 #include <string>
 
@@ skipping 951 matching lines @@
 
   const UIResourceClientData& data = iter->second;
   return data.size;
 }
 
 void LayerTreeHost::RegisterViewportLayers(
     scoped_refptr<Layer> overscroll_elasticity_layer,
     scoped_refptr<Layer> page_scale_layer,
     scoped_refptr<Layer> inner_viewport_scroll_layer,
     scoped_refptr<Layer> outer_viewport_scroll_layer) {
+  DCHECK_IMPLIES(inner_viewport_scroll_layer,
+                 inner_viewport_scroll_layer != outer_viewport_scroll_layer);
   overscroll_elasticity_layer_ = overscroll_elasticity_layer;
   page_scale_layer_ = page_scale_layer;
   inner_viewport_scroll_layer_ = inner_viewport_scroll_layer;
   outer_viewport_scroll_layer_ = outer_viewport_scroll_layer;
 }
 
 void LayerTreeHost::RegisterSelection(const LayerSelection& selection) {
   if (selection_ == selection)
     return;
 
@@ skipping 246 matching lines @@
   return animation_host_ ? animation_host_->HasAnyAnimation(layer->id())
                          : false;
 }
 
 bool LayerTreeHost::HasActiveAnimation(const Layer* layer) const {
   return animation_host_ ? animation_host_->HasActiveAnimation(layer->id())
                          : false;
 }
 
 }  // namespace cc