Hook ScrollElasticityController to InputHandlerProxy

cc/trees/layer_tree_host_impl.h

Index: cc/trees/layer_tree_host_impl.h
diff --git a/cc/trees/layer_tree_host_impl.h b/cc/trees/layer_tree_host_impl.h
index 6e860bc805f31c929f49a780b2882d4512721fa3..1fc0942c4d7ae204c4603ce8e4d7bd57fe06fd40 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -46,6 +46,7 @@ class EvictionTilePriorityQueue;
 class FrameRateCounter;
 class LayerImpl;
 class LayerTreeImpl;
+class LayerTreeScrollElasticityClient;
 class MemoryHistory;
 class PageScaleAnimation;
 class PaintTimeCounter;
@@ -55,6 +56,7 @@ class RasterWorkerPool;
 class RenderPassDrawQuad;
 class RenderingStatsInstrumentation;
 class ResourcePool;
+class ScrollElasticityControllerClient;
 class ScrollbarLayerImplBase;
 class TextureMailboxDeleter;
 class TopControlsManager;
@@ -156,6 +158,8 @@ class CC_EXPORT LayerTreeHostImpl
   bool HaveTouchEventHandlersAt(const gfx::Point& viewport_port) override;
   scoped_ptr<SwapPromiseMonitor> CreateLatencyInfoSwapPromiseMonitor(
       ui::LatencyInfo* latency) override;
+  ScrollElasticityControllerClient* GetScrollElasticityControllerClient()
+      override;
 
   // TopControlsManagerClient implementation.
   void SetControlsTopOffset(float offset) override;
@@ -626,6 +630,10 @@ class CC_EXPORT LayerTreeHostImpl
   int scroll_layer_id_when_mouse_over_scrollbar_;
   ScopedPtrVector<SwapPromise> swap_promises_for_main_thread_scroll_update_;
 
+  // An object to implement the ScrollElasticityControllerClient interface and
+  // hold all state related to elasticity. May be NULL if never requested.
+  scoped_ptr<LayerTreeScrollElasticityClient> scroll_elasticity_client_;
+
   bool tile_priorities_dirty_;
 
   // The optional delegate for the root layer scroll offset.