Gardening: media/encrypted-media/encrypted-media-waiting-for-a-key.html flaky

BUG=353792
TEST=
TBR=zmo@chromium.org

Review URL: https://codereview.chromium.org/203493006

git-svn-id: svn://svn.chromium.org/blink/trunk@169498 bbb929c8-8fbe-4397-9dbb-9b2b20218538