Fix Clock offset bug.

Stopping and then restarting a WebCore::Clock resulted in junk values on subsequent currentTime() calls, this caused MediaController (the only user of Clock as far as I can tell) to lose the playback position when paused.

BUG=242839

Review URL: https://codereview.chromium.org/27222006

git-svn-id: svn://svn.chromium.org/blink/trunk@159814 bbb929c8-8fbe-4397-9dbb-9b2b20218538