Remove the index of in-band tracks on the Chromium side

Note: https://codereview.chromium.org/754963009 removes the usage of
textTrackIndex() and has to be applied first.

BUG=440777
R=philipj@opera.com

Review URL: https://codereview.chromium.org/787343003

Cr-Commit-Position: refs/heads/master@{#308063}
4 files changed