Consume a user gesture when entering fullscreen.

BUG=304547
R=abarth@chromium.org
TEST=fullscreen/full-screen-enter-leave-enter.html

Review URL: https://codereview.chromium.org/26143002

git-svn-id: svn://svn.chromium.org/blink/trunk@158993 bbb929c8-8fbe-4397-9dbb-9b2b20218538