Merge 141904

Merge 141904
> [chromium] Make overlay layers slow-scrolling
> https://bugs.webkit.org/show_bug.cgi?id=108957
> 
> Patch by Sami Kyostila <skyostil@chromium.org>; on 2013-02-05
> Reviewed by James Robinson.
> 
> Since overlay layers get inserted on top of everything else, we must
> mark them slow-scrolling to prevent all scroll input events to the rest
> of the page from getting blocked. This is also more correct because
> generally the overlay contents need to be repainted whenever the scroll
> offset changes, and with this patch the painting happens in sync with
> page scrolling.

TBR=commit-queue@webkit.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=142275