WebKit2 View Gestures (Zoom): Can show unpainted regions when zoom-pinching-out

WebKit2 View Gestures (Zoom): Can show unpainted regions when zoom-pinching-out
https://bugs.webkit.org/show_bug.cgi?id=133472
<rdar://problem/15686848>

Reviewed by Simon Fraser.

* WebProcess/WebPage/mac/TiledCoreAnimationDrawingArea.h:
* WebProcess/WebPage/mac/TiledCoreAnimationDrawingArea.mm:
(WebKit::TiledCoreAnimationDrawingArea::flushLayers):
(WebKit::TiledCoreAnimationDrawingArea::applyTransientZoomToLayers):
(WebKit::TiledCoreAnimationDrawingArea::adjustTransientZoom):
Use prepopulateRect() when zooming out to ensure that we have tiles for the current exposed area.

Factor applyTransientZoomToLayers out of adjustTransientZoom, so that
flushLayers can call applyTransientZoomToLayers without ending up calling prepopulateRect()
during layer flushing, which causes trouble.

It is OK to populate layers at the current scale even when zooming out, because the overdraw
already covers most of the zoomed out area, and we're only adding a few more tiles at worst.

* WebCore.exp.in:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@169576 268f45cc-cd09-0410-ab3c-d52691b4dbfc