Bug 1367765 - Propagate the mHandledByAPZ flag when dispatching a touch event to the DOM. r=rhunt

This ensures that, if the touch event is over a scrollbar thumb and
makes it into nsSliderFrame::StartAPZDrag(), nsSliderFrame knows
that the event went through APZ and that therefore APZ will handle
the drag.

MozReview-Commit-ID: 92wAc1l9Pqz

--HG--
extra : rebase_source : 94ac60bc8b38dad3d8abaa39b5a94de88ec0f6b0

layout/base/PresShell.cpp

@@ -8464,6 +8464,7 @@ PresShell::DispatchTouchEventToDOM(WidgetEvent* aEvent,
                               touchEvent->mMessage, touchEvent->mWidget);
     newEvent.AssignTouchEventData(*touchEvent, false);
     newEvent.mTarget = targetPtr;
+    newEvent.mFlags.mHandledByAPZ = touchEvent->mFlags.mHandledByAPZ;
 
     RefPtr<PresShell> contentPresShell;
     if (doc == mDocument) {