[Mac][WebKit2] fast/css/sticky/sticky-top-zoomed.html is flaky on Mountain Lion

Unreviewed.

Patch by Benjamin Poulain <bpoulain@apple.com> on 2013-04-08

* platform/mac-wk2/TestExpectations:
* platform/mac/TestExpectations:
The test is flaky on MountainLion for WebKit1 and WebKit2. It fails systematically
in Debug or on older OS X.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147970 268f45cc-cd09-0410-ab3c-d52691b4dbfc