Make LayerTreeImpl::ScrollableArea more robust

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 "base/debug/trace_event.h"
 #include "cc/animation/animation.h"
 #include "cc/animation/animation_id_provider.h"
 #include "cc/animation/keyframed_animation_curve.h"
@@ skipping 340 matching lines @@
   return true;
 }
 
 const LayerImplList& LayerTreeImpl::RenderSurfaceLayerList() const {
   // If this assert triggers, then the list is dirty.
   DCHECK(!needs_update_draw_properties_);
   return render_surface_layer_list_;
 }
 
 gfx::Size LayerTreeImpl::ScrollableSize() const {
-  if (!root_scroll_layer_ || root_scroll_layer_->children().empty())
+  if (!root_scroll_layer_)
     return gfx::Size();
-  return root_scroll_layer_->children()[0]->bounds();
+
+  gfx::Size scrollable_size;
+  for (size_t i = 0; i < root_scroll_layer_->children().size(); ++i)
+      scrollable_size.ClampToMin(root_scroll_layer_->children()[i]->bounds());
+
+  return scrollable_size;
 }
 
 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 306 matching lines @@
   gfx::Vector2dF original_viewport_size =
       gfx::RectF(root_clip->bounds()).bottom_right() -
       gfx::PointF();
   original_viewport_size.Scale(1 / page_scale_factor());
 
   root_scroll->SetFixedContainerSizeDelta(
       scrollable_viewport_size - original_viewport_size);
 }
 
 }  // namespace cc