[CSS Regions] Fix positioning composited layers when the region has overflow:hidden

https://bugs.webkit.org/show_bug.cgi?id=124042

Patch by Mihai Maerean <mmaerean@adobe.com> on 2014-01-23
Reviewed by Mihnea Ovidenie.

Source/WebCore:

If there's a clipping GraphicsLayer on the hierarchy, substract its offset, since it's its
parent that positions us.

Tests: compositing/regions/position-layer-inside-region-overflow-hidden.html
       compositing/regions/position-layer-inside-overflow-hidden.html
       compositing/regions/position-layers-inside-region-overflow-hidden.html
       compositing/regions/position-layers-inside-regions-overflow-hidden.html

* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::adjustAncestorCompositingBoundsForFlowThread): The position
must also be correct when the region has box-shadow that inflates the region's layer. The
composited layers from the flow thread should be rendered in the same position whether the
associated region has clipping or not.
Using the position of the clipping layer instead of the location of the clipbox makes it
also work with box-shadow that inflates the region's graphics layer.

LayoutTests:

* compositing/regions/position-layer-inside-region-overflow-hidden.html: Added.
* compositing/regions/position-layer-inside-region-overflow-hidden-expected.html: Added.
* compositing/regions/position-layer-inside-overflow-hidden.html: Added.
* compositing/regions/position-layer-inside-overflow-hidden-expected.html: Added.
* compositing/regions/position-layers-inside-region-overflow-hidden.html: Added.
* compositing/regions/position-layers-inside-region-overflow-hidden-expected.html: Added.
* compositing/regions/position-layers-inside-regions-overflow-hidden.html: Added.
* compositing/regions/position-layers-inside-regions-overflow-hidden-expected.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@162605 268f45cc-cd09-0410-ab3c-d52691b4dbfc