REGRESSION (r238357): Pins on Yelp map disappear

REGRESSION (r238357): Pins on Yelp map disappear
https://bugs.webkit.org/show_bug.cgi?id=192597
rdar://problem/46578285

Reviewed by Zalan Bujtas.
Source/WebCore:

RenderLayerCompositor::updateBackingAndHierarchy() had a bug where if a RenderLayer gained
a negative z-order child (triggering creation of a foreground layer), we'd fail to
call the "setChildren()" with the vector containing that foreground layer.

When updateBackingAndHierarchy() stops visiting descendants because none are composited,
it may still have to update the child list with the foreground layer, so make sure
the code handles this case.

Tests: compositing/z-order/add-negative-z-child.html
       compositing/z-order/rebuild-sibling-of-layer-with-foreground-layer.html

* rendering/RenderLayer.cpp:
(WebCore::outputPaintOrderTreeRecursive):
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::updateBackingAndHierarchy):

LayoutTests:

Add tests that toggle negative z-index on a child, with and without sibling compositing layers.

* compositing/z-order/add-negative-z-child-expected.html: Added.
* compositing/z-order/add-negative-z-child.html: Added.
* compositing/z-order/rebuild-sibling-of-layer-with-foreground-layer-expected.html: Added.
* compositing/z-order/rebuild-sibling-of-layer-with-foreground-layer.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@239146 268f45cc-cd09-0410-ab3c-d52691b4dbfc