WebLayer.h cleanup

- Removes unused include WebPrivatePtr.h.
- Makes setCompositingReasons pure virtual (the Chromium code does implement this now)
- Removes transferAnimationsTo (implementation and calls were removed months ago in http://crbug.com/245520, but no followup patch to remove the declaration arrived)
- Makes setTouchEventHandlerRegion and touchEventHandlerRegion pure virtual (consistent with surrounding declarations; implementation has landed in Chromium)

BUG=

Review URL: https://chromiumcodereview.appspot.com/22982012

git-svn-id: svn://svn.chromium.org/blink/trunk@156173 bbb929c8-8fbe-4397-9dbb-9b2b20218538