Fix flakiness in cross-fade tests

A handful of cross-fade tests were flaky failures due to running before the images were loaded. Coincidentally, this problem was exacerbated by the switch to using the pause API as opposed to rAF since it allowed the tests to run immediately.

Updating the tests to wait for images to be loaded before running.

BUG=248938,249595

Review URL: https://codereview.chromium.org/26325005

git-svn-id: svn://svn.chromium.org/blink/trunk@159725 bbb929c8-8fbe-4397-9dbb-9b2b20218538