[Mac] Fix test expectations for ietestcenter/css3/grid/grid-items-003.htm

Unreviewed.

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

* platform/mac-wk2/TestExpectations: The test pass on WebKit2 Mountain Lion Release builds.
* platform/mac/TestExpectations: The test is flaky on Mountain Lion Release builds.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147971 268f45cc-cd09-0410-ab3c-d52691b4dbfc