Mark full-screen-video-has-backdrop.html as flaky.

Unreviewed gardening.

BUG=340351
TBR=falken@chromium.org

Review URL: https://codereview.chromium.org/145483007

git-svn-id: svn://svn.chromium.org/blink/trunk@166326 bbb929c8-8fbe-4397-9dbb-9b2b20218538