Make LayerTreeImpl::ScrollableArea more robust

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index eadc4043ab43621b06a0cc02fd7f4d000651856b..c24e314f06b29946987f884639684d1213de3673 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -358,9 +358,14 @@ const LayerImplList& LayerTreeImpl::RenderSurfaceLayerList() const {
 }
 
 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) {