[WK2] Pinch-zoom shadows can overlap headers and footers

[WK2] Pinch-zoom shadows can overlap headers and footers
https://bugs.webkit.org/show_bug.cgi?id=134372
<rdar://problem/16004095>

Reviewed by Simon Fraser.

* WebProcess/WebPage/mac/TiledCoreAnimationDrawingArea.mm:
(WebKit::shadowLayerPositionForFrame):
(WebKit::shadowLayerBoundsForFrame):
Compute the initial shadow layer and shadow bounds exactly as RenderLayerCompositor does
(in updateRootLayerPosition and friends). Also, clip the shadow layer to the old document rect,
otherwise it can extend over top of the header/footers. The content is automatically clipped to
this rect by the root content layer, but the shadow is outside of that.

(WebKit::TiledCoreAnimationDrawingArea::applyTransientZoomToLayers):
(WebKit::TiledCoreAnimationDrawingArea::commitTransientZoom):
Pass all of the arguments to constrainScrollPositionForOverhang, not just half of them.
There's still a bug where we constrain incorrectly with header/footer layers, but that
will be addressed elsewhere.

(WebKit::TiledCoreAnimationDrawingArea::applyTransientZoomToPage):
Make use of the new helpers.

* WebCore.exp.in:
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::updateLayerForHeader):
(WebCore::RenderLayerCompositor::updateLayerForFooter):
Put header/footer layers in front of the root content layer instead of behind.
Ideally nothing would ever overlap them, but this will give them precedence regardless.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@170514 268f45cc-cd09-0410-ab3c-d52691b4dbfc