Fix invalidating the text track indexes after append or remove text track from the list

There was a trivial bug in 'for' loop which iterates through the array
indexes but always uses the initialisation value of the iterator instead of
real value of the iterator.

BUG=

Review-Url: https://codereview.chromium.org/2144543002
Cr-Commit-Position: refs/heads/master@{#405709}
3 files changed