Scrollbars don't show when scrolling on the scrolling thread

Scrollbars don't show when scrolling on the scrolling thread
https://bugs.webkit.org/show_bug.cgi?id=77396
<rdar://problem/10710736>

Reviewed by Sam Weinig.

Use ScrollAnimator::scrollToOffsetWithoutAnimation when updating the frame view scroll offset,
since that function will end up invalidating the scrollbars correctly.

* page/ScrollingCoordinator.cpp:
(WebCore::ScrollingCoordinator::didUpdateMainFrameScrollPosition):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106317 268f45cc-cd09-0410-ab3c-d52691b4dbfc