Remove useless document NULL check in HTMLVideoElement::supportsFullscreen()

Remove useless document NULL check in HTMLVideoElement::supportsFullscreen().
It can never be NULL as document() returns a reference.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23872002

git-svn-id: svn://svn.chromium.org/blink/trunk@157086 bbb929c8-8fbe-4397-9dbb-9b2b20218538