Bug 1461708 - part 4: Move implementation of UIEvent::GetRangeParent() and UIEvent::RangeOffset() to nsLayoutUtils r=smaug

We need to move EditorEventListener::HandleMiddleClickPaste() into
EventStateManager to handle middle click paste after all click events are
dispatched.  This is preparation of the change.

HandleMiddleClickPaste() uses UIEvent::GetRangeParent() and
UIEvent::RangeOffset() to collapse Selection at clicked point.  However,
EventStateManager cannot access them since EventStateManager can handle it
with WidgetMouseEvent.  Fortunately, only WidgetMouseEvent is necessary for
implementing them.  Therefore, we can move the implementation into
nsLayoutUtils and merge them.

Differential Revision: https://phabricator.services.mozilla.com/D7851

--HG--
extra : moz-landing-system : lando