[CSS Regions] Activate all regions to have layers, as CSS Regions create
a new stacking context

The CSS Regions specification says "CSS Regions create a new stacking
context." (http://dev.w3.org/csswg/css-regions/).

Divs that create stacking contexts are painted after the divs that don't
create stacking contexts.

For the ref tests, the divs in the expected results corresponding to the
regions now have a CSS style causing them to also create Stacking
Contexts.

While the region-sibling-paint-order.html test verifies that regions
paint correctly when the next sibling also creates a stacking context,
tacking-context-paint-order.html verifies that regions paint correctly
when the next sibling doesn't create a stacking context.

Ported from http://wkb.ug/121828

The patch also includes the following fixes by Mihnea Ovidenie that are
related to the issue (CSS Regions creating Stacking Contexts):
http://wkb.ug/122405 Computed z-Index should return 0 instead of auto
for a region
http://wkb.ug/122389 Regions with overflow: hidden should paint over
positioned sibling
http://wkb.ug/122321 Regions as stacking contexts should paint over
positioned sibling
http://wkb.ug/122543 Use RenderStyle::hasStyleRegion when needed

BUG=303026

Review URL: https://codereview.chromium.org/25877003

git-svn-id: svn://svn.chromium.org/blink/trunk@159584 bbb929c8-8fbe-4397-9dbb-9b2b20218538