Having this event means that we don't have to wait for content to find out whether it's scrollable in the case that no swipe should be happening anyway.
IGNORE IDL because I'm only changing comments in the IDL files.
--HG--
extra : commitid : 22814fSLcIz
extra : rebase_source : e51b4db26333275c9bcb3fd7fece78b7d3653275