cc: Use scroll layer bounds as scrollable area size

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 <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 707 matching lines @@
     return gfx::SizeF();
 
   return gfx::ScaleSize(InnerViewportContainerLayer()->BoundsForScrolling(),
                         1.0f / current_page_scale_factor());
 }
 
 gfx::Rect LayerTreeImpl::RootScrollLayerDeviceViewportBounds() const {
   LayerImpl* root_scroll_layer = OuterViewportScrollLayer()
                                      ? OuterViewportScrollLayer()
                                      : InnerViewportScrollLayer();
-  if (!root_scroll_layer || root_scroll_layer->children().empty())
+  if (!root_scroll_layer)
     return gfx::Rect();
-  LayerImpl* layer = root_scroll_layer->children()[0];
-  return MathUtil::MapEnclosingClippedRect(layer->ScreenSpaceTransform(),
-                                           gfx::Rect(layer->bounds()));
+  return MathUtil::MapEnclosingClippedRect(
+      root_scroll_layer->ScreenSpaceTransform(),
+      gfx::Rect(root_scroll_layer->bounds()));
 }
 
 void LayerTreeImpl::ApplySentScrollAndScaleDeltasFromAbortedCommit() {
   DCHECK(IsActiveTree());
 
   page_scale_factor()->AbortCommit();
   top_controls_shown_ratio()->AbortCommit();
   elastic_overscroll()->AbortCommit();
 
   if (!root_layer())
@@ skipping 238 matching lines @@
   // If this assert triggers, then the render_surface_layer_list_ is dirty, so
   // the unoccluded_screen_space_region_ is not valid anymore.
   DCHECK(!needs_update_draw_properties_);
   return unoccluded_screen_space_region_;
 }
 
 gfx::SizeF LayerTreeImpl::ScrollableSize() const {
   LayerImpl* root_scroll_layer = OuterViewportScrollLayer()
                                      ? OuterViewportScrollLayer()
                                      : InnerViewportScrollLayer();
-  if (!root_scroll_layer || root_scroll_layer->children().empty())
+  if (!root_scroll_layer)
     return gfx::SizeF();
 
-  gfx::SizeF content_size =
-      root_scroll_layer->children()[0]->BoundsForScrolling();
+  gfx::SizeF content_size = root_scroll_layer->BoundsForScrolling();
   gfx::SizeF viewport_size =
       root_scroll_layer->scroll_clip_layer()->BoundsForScrolling();
 
   content_size.SetToMax(viewport_size);
   return content_size;
 }
 
 LayerImpl* LayerTreeImpl::LayerById(int id) const {
   LayerImplMap::const_iterator iter = layer_id_map_.find(id);
   return iter != layer_id_map_.end() ? iter->second : NULL;
@@ skipping 1088 matching lines @@
 }
 
 void LayerTreeImpl::ResetAllChangeTracking(PropertyTrees::ResetFlags flag) {
   layers_that_should_push_properties_.clear();
   for (auto* layer : *this)
     layer->ResetChangeTracking();
   property_trees_.ResetAllChangeTracking(flag);
 }
 
 }  // namespace cc