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

cc/trees/layer_tree_host.h

 // 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.
 
 #ifndef CC_TREES_LAYER_TREE_HOST_H_
 #define CC_TREES_LAYER_TREE_HOST_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 456 matching lines @@
 
   MicroBenchmarkController micro_benchmark_controller_;
 
   void OnCommitForSwapPromises();
 
   void RecordGpuRasterizationHistogram();
 
  private:
   friend class LayerTreeHostSerializationTest;
 
+  void ApplyViewportDeltas(ScrollAndScaleSet* info);
   void InitializeProxy(
       std::unique_ptr<Proxy> proxy,
       std::unique_ptr<BeginFrameSource> external_begin_frame_source);
 
   bool DoUpdateLayers(Layer* root_layer);
   void UpdateHudLayer();
 
   bool AnimateLayersRecursive(Layer* current, base::TimeTicks time);
 
   struct UIResourceClientData {
@@ skipping 113 matching lines @@
 
   uint32_t surface_id_namespace_;
   uint32_t next_surface_sequence_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHost);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_H_