Keep expanded if mouse moves off of scrollbar while dragging

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 20a08a44822ea73d5cbc11a6384d5c5d9a2d86c3..27cc7ed5f41e21548989a6f82d8f7f510f809d52 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -178,7 +178,11 @@ class CC_EXPORT LayerTreeHostImpl
       const gfx::ScrollOffset& root_offset) override;
   void ScrollEnd(ScrollState* scroll_state) override;
   InputHandler::ScrollStatus FlingScrollBegin() override;
+
+  void MouseDownAt(const gfx::Point& viewport_point) override;
+  void MouseUp() override;
   void MouseMoveAt(const gfx::Point& viewport_point) override;
+
   void PinchGestureBegin() override;
   void PinchGestureUpdate(float magnify_delta,
                           const gfx::Point& anchor) override;
@@ -844,6 +848,8 @@ class CC_EXPORT LayerTreeHostImpl
   std::unique_ptr<PendingTreeDurationHistogramTimer>
       pending_tree_duration_timer_;
 
+  int captured_scrollbar_layer_id_;
+
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };