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 122 matching lines @@
 
 LayerTreeHostCommon::CalcDrawPropsImplInputsForTesting::
     CalcDrawPropsImplInputsForTesting(LayerImpl* root_layer,
                                       const gfx::Size& device_viewport_size,
                                       LayerImplList* render_surface_layer_list)
     : CalcDrawPropsImplInputsForTesting(root_layer,
                                         device_viewport_size,
                                         gfx::Transform(),
                                         render_surface_layer_list) {}
 
+LayerTreeHostCommon::ScrollUpdateInfo::ScrollUpdateInfo()
+    : layer_id(Layer::INVALID_ID) {}
+
 bool LayerTreeHostCommon::ScrollUpdateInfo::operator==(
     const LayerTreeHostCommon::ScrollUpdateInfo& other) const {
   return layer_id == other.layer_id && scroll_delta == other.scroll_delta;
 }
 
 void LayerTreeHostCommon::ScrollUpdateInfo::ToProtobuf(
     proto::ScrollUpdateInfo* proto) const {
   proto->set_layer_id(layer_id);
   Vector2dToProto(scroll_delta, proto->mutable_scroll_delta());
 }
@@ skipping 536 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