Move viewport scrolling logic into separate class

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 3105e6f48eaa6c0cd0eb2c380d9285495aaa4059..e33737bdbb1c9c3de5f923a1343a1bd13ff4ba60 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -44,6 +44,7 @@ class Proxy;
 class ResourceProvider;
 class TileManager;
 class UIResourceRequest;
+class Viewport;
 struct PendingPageScaleAnimation;
 struct RendererCapabilities;
 struct SelectionHandle;
@@ -329,6 +330,8 @@ class CC_EXPORT LayerTreeImpl {
       scoped_ptr<PendingPageScaleAnimation> pending_animation);
   scoped_ptr<PendingPageScaleAnimation> TakePendingPageScaleAnimation();
 
+  Viewport* viewport() { return viewport_.get(); }
+
  protected:
   explicit LayerTreeImpl(
       LayerTreeHostImpl* layer_tree_host_impl,
@@ -416,6 +419,8 @@ class CC_EXPORT LayerTreeImpl {
 
   scoped_ptr<PendingPageScaleAnimation> pending_page_scale_animation_;
 
+  scoped_ptr<Viewport> viewport_;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(LayerTreeImpl);
 };