Allow scrolling a page with selection after long-press on whitespace

ui/touch_selection/longpress_drag_selector.h

Index: ui/touch_selection/longpress_drag_selector.h
diff --git a/ui/touch_selection/longpress_drag_selector.h b/ui/touch_selection/longpress_drag_selector.h
index f6e84ecf1cd7463786720c79dd3779f383134cb3..4ebdd675a875d489b13a66569f0b04804586b29d 100644
--- a/ui/touch_selection/longpress_drag_selector.h
+++ b/ui/touch_selection/longpress_drag_selector.h
@@ -40,6 +40,9 @@ class UI_TOUCH_SELECTION_EXPORT LongPressDragSelector
   void OnLongPressEvent(base::TimeTicks event_time,
                         const gfx::PointF& position);
 
+  // Called when a scroll is going to happen to cancel longpress-drag gesture.
+  void OnScrollBeginEvent();
+
   // Called when the active selection changes.
   void OnSelectionActivated();
   void OnSelectionDeactivated();