Remove HTMLMediaElement.initialTime

It was dropped from spec in April 2012:
http://html5.org/r/7046

BUG=309474

Review URL: https://codereview.chromium.org/35033002

git-svn-id: svn://svn.chromium.org/blink/trunk@160209 bbb929c8-8fbe-4397-9dbb-9b2b20218538