Unbreak max scroll offset in threaded compositing mode.

cc/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/layer_tree_impl.h"
 
 #include "base/debug/trace_event.h"
 #include "cc/layer_tree_host_common.h"
 #include "cc/layer_tree_host_impl.h"
 #include "ui/gfx/vector2d_conversions.h"
@@ skipping 170 matching lines @@
   return true;
 }
 
 const LayerTreeImpl::LayerList& LayerTreeImpl::RenderSurfaceLayerList() const {
   // If this assert triggers, then the list is dirty.
   DCHECK(!layer_tree_host_impl_->needsUpdateDrawProperties());
   return render_surface_layer_list_;
 }
 
 gfx::Size LayerTreeImpl::ContentSize() const {
-  if (!root_scroll_layer_)
+  if (!root_scroll_layer_ || root_scroll_layer_->children().empty())
     return gfx::Size();
-  return root_scroll_layer_->bounds();
+  return root_scroll_layer_->children()[0]->bounds();
 }
 
 LayerImpl* LayerTreeImpl::LayerById(int id) {
   LayerIdMap::iterator iter = layer_id_map_.find(id);
   return iter != layer_id_map_.end() ? iter->second : NULL;
 }
 
 void LayerTreeImpl::RegisterLayer(LayerImpl* layer) {
   DCHECK(!LayerById(layer->id()));
   layer_id_map_[layer->id()] = layer;
@@ skipping 125 matching lines @@
 
 AnimationRegistrar* LayerTreeImpl::animationRegistrar() const {
   return layer_tree_host_impl_->animationRegistrar();
 }
 
 const PinchZoomViewport& LayerTreeImpl::pinch_zoom_viewport() const {
   return layer_tree_host_impl_->pinchZoomViewport();
 }
 
 } // namespace cc