Route Scroll events through a cc::InputHandler

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 2e5b954ba3853611a1ca9b042aa394ca5c45c49b..95307c4dc154b1d95f17e28dc5822f6e6435024d 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2957,6 +2957,7 @@ void LayerTreeHostImpl::ApplyScroll(ScrollNode* scroll_node,
   // we want to scroll the inner viewport -- to allow panning while zoomed --
   // but also move browser controls if needed.
   bool is_inner_viewport_scroll_layer =
+      InnerViewportScrollLayer() &&
       scroll_node->owner_id == InnerViewportScrollLayer()->id();
 
   if (is_viewport_scroll_layer || is_inner_viewport_scroll_layer) {
@@ -3051,6 +3052,21 @@ InputHandlerScrollResult LayerTreeHostImpl::ScrollBy(
   DCHECK(scroll_state);
 
   TRACE_EVENT0("cc", "LayerTreeHostImpl::ScrollBy");
+
+  if (!CurrentlyScrollingLayer()) {
+    // If the InputHandlerClient didn't call ScrollBegin() to re-perform a
+    // hit-test but provided an element to scroll instead, scroll that.
+    ElementId provided_element =
+        scroll_state->data()->current_native_scrolling_element();
+    if (provided_element) {
+      // Assume the primaryId matches the layer_id for lookup (this is true for
+      // ui:Layers), then check the match.
+      LayerImpl* layer = active_tree()->LayerById(provided_element.primaryId);
+      if (layer && layer->element_id() == provided_element)
+        active_tree_->SetCurrentlyScrollingLayer(layer);
+    }
+  }
+
   if (!CurrentlyScrollingLayer())
     return InputHandlerScrollResult();