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

cc/trees/layer_tree_impl.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_impl.h"
 
 #include <algorithm>
 #include <limits>
 #include <set>
 
@@ skipping 247 matching lines @@
              ? OuterViewportScrollLayer()->scroll_clip_layer()
              : NULL;
 }
 
 LayerImpl* LayerTreeImpl::CurrentlyScrollingLayer() const {
   DCHECK(IsActiveTree());
   return LayerById(currently_scrolling_layer_id_);
 }
 
 void LayerTreeImpl::SetCurrentlyScrollingLayer(LayerImpl* layer) {
+  DCHECK_IMPLIES(layer, layer != OuterViewportScrollLayer());
+
   int new_id = layer ? layer->id() : Layer::INVALID_ID;
   if (currently_scrolling_layer_id_ == new_id)
     return;
 
   if (CurrentlyScrollingLayer() &&
       CurrentlyScrollingLayer()->scrollbar_animation_controller())
     CurrentlyScrollingLayer()->scrollbar_animation_controller()->DidScrollEnd();
   currently_scrolling_layer_id_ = new_id;
   if (layer && layer->scrollbar_animation_controller())
     layer->scrollbar_animation_controller()->DidScrollBegin();
@@ skipping 1430 matching lines @@
                                                    const gfx::BoxF& box,
                                                    gfx::BoxF* bounds) const {
   *bounds = gfx::BoxF();
   return layer_tree_host_impl_->animation_host()
              ? layer_tree_host_impl_->animation_host()
                    ->TransformAnimationBoundsForBox(layer->id(), box, bounds)
              : true;
 }
 
 }  // namespace cc