Add WD EME ClearKey playback test.

BUG=338485
TEST=This CL adds a new layout test.

Review URL: https://codereview.chromium.org/148853003

git-svn-id: svn://svn.chromium.org/blink/trunk@166971 bbb929c8-8fbe-4397-9dbb-9b2b20218538