Move overscroll bounce to impl thread

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 157fe00eb67013347d1840a7c9499e392b885fcf..af398ef1a3ff8c21bfc917e7c5d306785ce221e5 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -57,6 +57,7 @@ class RenderPassDrawQuad;
 class RenderingStatsInstrumentation;
 class ResourcePool;
 class ScrollbarLayerImplBase;
+class TreeScrollElasticityClient;
 class TextureMailboxDeleter;
 class TopControlsManager;
 class UIResourceBitmap;
@@ -156,6 +157,8 @@ 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;
@@ -627,6 +630,11 @@ class CC_EXPORT LayerTreeHostImpl
   int scroll_layer_id_when_mouse_over_scrollbar_;
   ScopedPtrVector<SwapPromise> swap_promises_for_main_thread_scroll_update_;
 
+  // The LayerTreeHostImpl class could implement the
+  // ScrollElasticityControllerClient interface itself, but an interface
+  // class is created to keep related state and functions separate.
+  scoped_ptr<TreeScrollElasticityClient> scroll_elasticity_client_;
+
   bool tile_priorities_dirty_;
 
   // The optional delegate for the root layer scroll offset.