Remove HTMLMediaElement::minTimeSeekable()/maxTimeSeekable()

minTimeSeekable() is completely unused, and maxTimeSeekable()
was only used in a single place, so just inline it there.

BUG=none

Review URL: https://codereview.chromium.org/68183003

git-svn-id: svn://svn.chromium.org/blink/trunk@161785 bbb929c8-8fbe-4397-9dbb-9b2b20218538