Keep expanded if mouse moves off of scrollbar while dragging

ui/events/blink/input_handler_proxy.cc

Index: ui/events/blink/input_handler_proxy.cc
diff --git a/ui/events/blink/input_handler_proxy.cc b/ui/events/blink/input_handler_proxy.cc
index b4e93ae46fbfcf47450ed2a6763d6c2d8d2fcf14..6c0254435dd48bb905b7838e99d3225488ab90f0 100644
--- a/ui/events/blink/input_handler_proxy.cc
+++ b/ui/events/blink/input_handler_proxy.cc
@@ -373,10 +373,23 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleInputEvent(
     case WebInputEvent::TouchEnd:
       return HandleTouchEnd(static_cast<const WebTouchEvent&>(event));
 
+    case WebInputEvent::MouseDown: {
+      // Only for check scrollbar captured
+      const WebMouseEvent& mouse_event =
+          static_cast<const WebMouseEvent&>(event);
+      CHECK(input_handler_);
+      input_handler_->MouseDownAt(gfx::Point(mouse_event.x, mouse_event.y));
+      return DID_NOT_HANDLE;
+    }
+    case WebInputEvent::MouseUp: {
+      // Only for release scrollbar captured
+      CHECK(input_handler_);
+      input_handler_->MouseUp();
+      return DID_NOT_HANDLE;
+    }
     case WebInputEvent::MouseMove: {
       const WebMouseEvent& mouse_event =
           static_cast<const WebMouseEvent&>(event);
-      // TODO(tony): Ignore when mouse buttons are down?
       // TODO(davemoore): This should never happen, but bug #326635 showed some
       // surprising crashes.
       CHECK(input_handler_);