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 9ac94ba6e7dcc685129a0646cc4e6e6d8bb4736a..3c93374ef6b0866327741de24210d0753b3b7734 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -176,7 +176,11 @@ 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;
@@ -846,6 +850,8 @@ class CC_EXPORT LayerTreeHostImpl
   std::unique_ptr<PendingTreeDurationHistogramTimer>
       pending_tree_duration_timer_;
 
+  int captured_scrollbar_layer_id_;

[weiliangc, 2016/09/29 21:37:33]

nit: Could you move this close to the scroll_layer_id_when_mouse_over_scrollbar_
to show they are somewhat related?

+
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };