Web Animations: guard testRunner usages.

When testing web animations test htmls on chrome, "Uncaught ReferenceError:
testRunner is not defined" exception is fired.
So guard testRunner for convenience.

BUG=N/A

Review URL: https://codereview.chromium.org/436883002

git-svn-id: svn://svn.chromium.org/blink/trunk@179917 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Avoid preloader-induced test flakiness.

If a preload request for <img> preload fails, the request will be retried
on parsing the element as per normal. This might result in more than
one request for the same resource, and consequently more than one console
error message might be output.

For img-crossorigin-no-credentials-prompt.html, avoid having the preloader
kick in by not relying on an external script.

R=
BUG=239918

Review URL: https://codereview.chromium.org/461563002

git-svn-id: svn://svn.chromium.org/blink/trunk@179918 bbb929c8-8fbe-4397-9dbb-9b2b20218538