Mark WebGLInfoBarTest.ContextLossInfoBarReload FLAKY on Mac

BUG=324555
TBR=zmo@chromium.org, danakj@chromium.org

Review URL: https://codereview.chromium.org/97473002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237968 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237968