Update the Mac WebKit2 TestExpectations for http/tests/canvas/canvas-slow-font-loading.html

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

* platform/mac-wk2/TestExpectations: The test is flaky in release. Mark it as such.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147959 268f45cc-cd09-0410-ab3c-d52691b4dbfc