Make it possible for the scroll offset delegate to intercept fling.

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 f97eea0f51f09f4ade0b5dc3043f1523d1bb9620..aeb77e47ea25771d9761bdf5e4816c4fd4f8b134 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2072,10 +2072,17 @@ void LayerTreeHostImpl::ScrollEnd() {
 }
 
 InputHandler::ScrollStatus LayerTreeHostImpl::FlingScrollBegin() {
-  if (active_tree_->CurrentlyScrollingLayer())
-    return ScrollStarted;
+  if (!active_tree_->CurrentlyScrollingLayer())
+    return ScrollIgnored;
 
-  return ScrollIgnored;
+  if (settings_.ignore_root_layer_flings &&
+      active_tree_->CurrentlyScrollingLayer() ==
+          active_tree_->RootScrollLayer()) {
+    ClearCurrentlyScrollingLayer();
+    return ScrollIgnored;
+  }
+
+  return ScrollStarted;
 }
 
 void LayerTreeHostImpl::NotifyCurrentFlingVelocity(gfx::Vector2dF velocity) {