Remove TextTrackClient interface to simplify TextTrack code.

Removing the TextTrackClient interface because it was obscuring calls
made to the HTMLMediaElement and wasn't really providing much value.
Calls made to the HTMLMediaElement are much clearer now and it is
easier to follow what happens when tracks are removed from the
TextTrackList.

BUG=249427
TEST=All existing LayoutTests still pass
R=adamk@chromium.org

Review URL: https://codereview.chromium.org/177003025

git-svn-id: svn://svn.chromium.org/blink/trunk@168534 bbb929c8-8fbe-4397-9dbb-9b2b20218538