Made layout viewport scroll updates from compositor work like ordinary layers.

cc/trees/layer_tree_host_common.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host_common.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 141 matching lines @@
 }
 
 void LayerTreeHostCommon::ScrollUpdateInfo::FromProtobuf(
     const proto::ScrollUpdateInfo& proto) {
   layer_id = proto.layer_id();
   scroll_delta = ProtoToVector2d(proto.scroll_delta());
 }
 
 ScrollAndScaleSet::ScrollAndScaleSet()
     : page_scale_delta(1.f), top_controls_delta(0.f) {
+  inner_viewport_scroll.layer_id = Layer::INVALID_ID;
 }
 
 ScrollAndScaleSet::~ScrollAndScaleSet() {}
 
 bool ScrollAndScaleSet::EqualsForTesting(const ScrollAndScaleSet& other) const {
   return scrolls == other.scrolls &&
          page_scale_delta == other.page_scale_delta &&
          elastic_overscroll_delta == other.elastic_overscroll_delta &&
          top_controls_delta == other.top_controls_delta;
 }
@@ skipping 517 matching lines @@
 
 PropertyTrees* GetPropertyTrees(Layer* layer) {
   return layer->layer_tree_host()->property_trees();
 }
 
 PropertyTrees* GetPropertyTrees(LayerImpl* layer) {
   return layer->layer_tree_impl()->property_trees();
 }
 
 }  // namespace cc