virtual/web-animations-css/animations/cross-fade-background-image.html is flaky TIMEOUT

Add a TIMEOUT TestExpectation for this test.

TBR=dstockwell@chromium.org
NOTRY=true
BUG=306967

Review URL: https://codereview.chromium.org/27164004

git-svn-id: svn://svn.chromium.org/blink/trunk@159576 bbb929c8-8fbe-4397-9dbb-9b2b20218538