Remove the mode getter/setter for in-band text tracks

TextTrack::setMode() is virtual and overridden by InbandTextTrack to
forward all the way down to Chromium's WebInbandTextTrackImpl, where
it is simply stored. However, the getter is not virtual and does not
ever retrieve the value, so all of this is elaborate dead code.

Because InbandTextTrack::setMode() first called TextTrack::setMode(),
this assymetry between getting and setting for in-band tracks didn't
actually break anything, and the relevant tests still pass.

In addition, in-band tracks should not need to know about the mode,
they simply provide cues for Blink, whether or not they are rendered
can be left an internal matter.

The Chromium side can be removed after a Blink roll, after which
WebInbandTextTrack::Mode can finally be removed as well.

BUG=315476
TEST=LayoutTests/media/track/opera/interfaces/TextTrack/mode.html

Review URL: https://codereview.chromium.org/60203008

git-svn-id: svn://svn.chromium.org/blink/trunk@161456 bbb929c8-8fbe-4397-9dbb-9b2b20218538