webkit_unit_test WebFrameCSSCallbackTest.InvalidSelector flaky on MacOS

BUG=310361
TBR=pfeldman@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/34523008

git-svn-id: svn://svn.chromium.org/blink/trunk@160269 bbb929c8-8fbe-4397-9dbb-9b2b20218538