Harden a fullscreen transition test

Since the test uses only timeouts and no events, it passes even if the
webkitRequestFullScreen() call fails, as it would e.g. if the
webkitallowfullscreen attribute were simply removed.

BUG=390008

Review URL: https://codereview.chromium.org/355343002

git-svn-id: svn://svn.chromium.org/blink/trunk@177196 bbb929c8-8fbe-4397-9dbb-9b2b20218538