[chromium] Remove deprecated flingStart.sourceDevice from WebGestureEvent
https://bugs.webkit.org/show_bug.cgi?id=111866

Reviewed by James Robinson.

* public/WebInputEvent.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145321 268f45cc-cd09-0410-ab3c-d52691b4dbfc