Fix for <rdar://problem/8946961> Fix repaint problems

Fix for <rdar://problem/8946961> Fix repaint problems
with WKScrollbarPainter scrollers while resizing the window

Reviewed by Sam Weinig.

Remove setPainterForPainterController() and
removePainterFromPainterController() since they aren't
used anymore.
* platform/mac/ScrollAnimatorMac.h:

Return early if they alpha hasn't changed.
* platform/mac/ScrollAnimatorMac.mm:
(-[ScrollbarPainterDelegate scrollerImp:animateKnobAlphaTo:duration:]):
(-[ScrollbarPainterDelegate scrollerImp:animateTrackAlphaTo:duration:]):

If we're in a live resize, restore alpha to 1.
(WebCore::ScrollAnimatorMac::didAddVerticalScrollbar):
(WebCore::ScrollAnimatorMac::didAddHorizontalScrollbar):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@77876 268f45cc-cd09-0410-ab3c-d52691b4dbfc