Tighten up preconditions for calling WebMediaPlayer::currentTime().

HTMLMediaElement shouldn't call WebMediaPlayer::currentTime() when
we HAVE_NOTHING. Doing so leaks spec implementation details into
lower levels of the stack.

Also remove some old crufty time-related code.

BUG=349543, 370634

Review URL: https://codereview.chromium.org/535703002

git-svn-id: svn://svn.chromium.org/blink/trunk@181329 bbb929c8-8fbe-4397-9dbb-9b2b20218538