Remove unfailable null checks in MediaControls

MediaControls::create returns null if any of these would be null.

BUG=341813
R=acolwell@chromium.org

Review URL: https://codereview.chromium.org/182383006

git-svn-id: svn://svn.chromium.org/blink/trunk@168474 bbb929c8-8fbe-4397-9dbb-9b2b20218538