Remove media seek back, seek forward, rewind, and return to realtime buttons.

Chromium's media controls implementation doesn't make use of these buttons.

BUG=337015
R=adamk@chromium.org, jamesr@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/146013005

git-svn-id: svn://svn.chromium.org/blink/trunk@165788 bbb929c8-8fbe-4397-9dbb-9b2b20218538