Keep expanded if mouse moves off of scrollbar while dragging

ui/events/blink/input_handler_proxy_unittest.cc

Index: ui/events/blink/input_handler_proxy_unittest.cc
diff --git a/ui/events/blink/input_handler_proxy_unittest.cc b/ui/events/blink/input_handler_proxy_unittest.cc
index d701dafb59421b2eb1fe0ac4806584ead59478ff..b26b5cf0977e2fcfc471b272326a191b07b5535a 100644
--- a/ui/events/blink/input_handler_proxy_unittest.cc
+++ b/ui/events/blink/input_handler_proxy_unittest.cc
@@ -137,6 +137,9 @@ class MockInputHandler : public cc::InputHandler {
 
   void BindToClient(cc::InputHandlerClient* client) override {}
 
+  void MouseDownAt(const gfx::Point& viewport_point) override {}
+  void MouseUp() override {}
+
   void MouseMoveAt(const gfx::Point& mouse_position) override {}
 
   MOCK_CONST_METHOD2(IsCurrentlyScrollingLayerAt,