Fix for <rdar://problem/8944544> Ability to animate track

Fix for <rdar://problem/8944544> Ability to animate track
for WKPainter scrollers

Reviewed by Sam Weinig.

Source/WebCore:

Two new WebKitSystemInterface functions.
* WebCore.exp.in:
* platform/mac/WebCoreSystemInterface.h:
* platform/mac/WebCoreSystemInterface.mm:

Use Scrollbar::convertFromContainingView() to return the right point.
* platform/mac/ScrollAnimatorMac.mm:
(-[ScrollbarPainterControllerDelegate scrollerImpPair:convertContentPoint:toScrollerImp:]):

ScrollKnobAnimation is now ScrollbarPartAnimation. It can
now be used to animate the knob or the track.
(-[ScrollbarPartAnimation initWithScrollbarPainter:part:WebCore::scrollAnimator:WebCore::animateAlphaTo:duration:]):
(-[ScrollbarPartAnimation setCurrentProgress:]):
(-[ScrollbarPainterDelegate setUpAnimation:scrollerPainter:part:WebCore::animateAlphaTo:duration:]):
(-[ScrollbarPainterDelegate scrollerImp:animateKnobAlphaTo:duration:]):
(-[ScrollbarPainterDelegate scrollerImp:animateTrackAlphaTo:duration:]):

Scrollbars need invalodating after the overlay state changes.
(-[ScrollbarPainterDelegate scrollerImp:overlayScrollerStateChangedTo:]):

Source/WebKit/mac:

* WebCoreSupport/WebSystemInterface.mm:
(InitWebCoreSystemInterface):

Source/WebKit2:

* WebCoreSupport/WebSystemInterface.mm:
(InitWebCoreSystemInterface):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@77555 268f45cc-cd09-0410-ab3c-d52691b4dbfc