Arrange compositor scrolling into scroll customization format

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index 973df9777339ddd913e421a8a934e68d74997b41..da30d6fa73a926bf8b3868af4bdface4207be30c 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -277,6 +277,10 @@ class CC_EXPORT LayerTreeImpl {
   // The outer viewport scroll layer scrolls first.
   void DistributeRootScrollOffset();
 
+  void ApplyScroll(LayerImpl* layer, ScrollState* scroll_state) {
+    layer_tree_host_impl_->ApplyScroll(layer, scroll_state);
+  }
+
   // Call this function when you expect there to be a swap buffer.
   // See swap_promise.h for how to use SwapPromise.
   void QueueSwapPromise(scoped_ptr<SwapPromise> swap_promise);