Don't set sent_top_controls_delta when root layer doesn't exist.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index a10d46b423c1e77b354494db0706179ec91ceea2..91ff5587bb6314ace51e01bc35bd6d392d19d995 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2904,8 +2904,7 @@ void LayerTreeHostImpl::PinchGestureEnd() {
 
 static void CollectScrollDeltas(ScrollAndScaleSet* scroll_info,
                                 LayerImpl* layer_impl) {
-  if (!layer_impl)
-    return;
+  DCHECK(layer_impl);
 
   gfx::Vector2d scroll_delta =
       gfx::ToFlooredVector2d(layer_impl->ScrollDelta());
@@ -2924,12 +2923,15 @@ static void CollectScrollDeltas(ScrollAndScaleSet* scroll_info,
 scoped_ptr<ScrollAndScaleSet> LayerTreeHostImpl::ProcessScrollDeltas() {
   scoped_ptr<ScrollAndScaleSet> scroll_info(new ScrollAndScaleSet());
 
-  CollectScrollDeltas(scroll_info.get(), active_tree_->root_layer());
-  scroll_info->page_scale_delta = active_tree_->page_scale_delta();
-  active_tree_->set_sent_page_scale_delta(scroll_info->page_scale_delta);
-  scroll_info->swap_promises.swap(swap_promises_for_main_thread_scroll_update_);
-  scroll_info->top_controls_delta = active_tree()->top_controls_delta();
-  active_tree_->set_sent_top_controls_delta(scroll_info->top_controls_delta);
+  if (active_tree_->root_layer()) {
+    CollectScrollDeltas(scroll_info.get(), active_tree_->root_layer());
+    scroll_info->page_scale_delta = active_tree_->page_scale_delta();
+    active_tree_->set_sent_page_scale_delta(scroll_info->page_scale_delta);
+    scroll_info->swap_promises.swap(
+        swap_promises_for_main_thread_scroll_update_);
+    scroll_info->top_controls_delta = active_tree()->top_controls_delta();
+    active_tree_->set_sent_top_controls_delta(scroll_info->top_controls_delta);
+  }
 
   return scroll_info.Pass();
 }