Defer scroll status determination until scrolling has occurred

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 4ccc5fe8b5e1304a6dd056da3fce7ee0de1daf67..3a6ce660b629457d62bb3241471a234226c69a8b 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1769,7 +1769,7 @@ LayerImpl* LayerTreeHostImpl::CurrentlyScrollingLayer() const {
 }
 
 bool LayerTreeHostImpl::IsCurrentlyScrolling() const {
-  return CurrentlyScrollingLayer() ||
+  return (did_lock_scrolling_layer_ && CurrentlyScrollingLayer()) ||
          (InnerViewportScrollLayer() &&
           InnerViewportScrollLayer()->IsExternalFlingActive()) ||
          (OuterViewportScrollLayer() &&
@@ -2408,7 +2408,6 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
     active_tree_->SetCurrentlyScrollingLayer(scrolling_layer_impl);
     should_bubble_scrolls_ = (type != NonBubblingGesture);
     wheel_scrolling_ = (type == Wheel);
-    client_->RenewTreePriority();

[brianderson, 2014/10/23 20:55:51]

Why is this removed?

Is it because we'll only have a scrolling layer at this point, but not have
actually scrolled?

[jdduke (slow), 2014/10/23 22:12:49]

Right, unless there's another side effect of this method that might cause a
different renewal priority? I guess it couldn't hurt to keep it? I'll add it
back.

     UMA_HISTOGRAM_BOOLEAN("TryScroll.SlowScroll", false);
     return ScrollStarted;
   }