Remove virtual where not needed in HTMLMediaElement/HTMLVideoElement

These functions were neither overrides or overridden anywhere, so
the virtual keywords were likely copypasta or unrealized plans.

BUG=313702

Review URL: https://codereview.chromium.org/53453004

git-svn-id: svn://svn.chromium.org/blink/trunk@161166 bbb929c8-8fbe-4397-9dbb-9b2b20218538