Encrypted Media: Add layout test for "waiting for a key" in WD EME spec.

Spec:
https://dvcs.w3.org/hg/html-media/raw-file/tip/encrypted-media/encrypted-media.html#waiting-for-a-key

BUG=338485

Review URL: https://codereview.chromium.org/164353003

git-svn-id: svn://svn.chromium.org/blink/trunk@167243 bbb929c8-8fbe-4397-9dbb-9b2b20218538