Use exitFullscreen(), not fullyExitFullscreen(), in HTMLMediaElement

For both <video controls> and HTMLVideoElement.webkitExitFullscreen()
it makes more sense to only exit one level of fullscreen.

There was no test coverage for the distinction, and it's very unlikely
that Web content could meaningfully depend on this.

BUG=399556

Review URL: https://codereview.chromium.org/422023007

git-svn-id: svn://svn.chromium.org/blink/trunk@179440 bbb929c8-8fbe-4397-9dbb-9b2b20218538