test_scroll_snapping_scrollbars.html is failing when apz.drag.enabled
is true because synthesizeMouse does not route events through APZ,
meaning that the drag is never handled by APZ or Gecko.
Resolve this by adding a check that mHandledByApz flag is true for
WidgetGUIEvent, and handle in Gecko if that is not the case.
MozReview-Commit-ID: IrxVLxqs64V