Hoist checks for 'disabled' tracks out of textTrackAddCue{,s}

In most cases the mode has already been checked by calling code, so the
check is redundant.
This is done in preparation of factoring the text track rendering code
out of HTMLMediaElement.

BUG=321654

Review URL: https://codereview.chromium.org/910843002

git-svn-id: svn://svn.chromium.org/blink/trunk@189944 bbb929c8-8fbe-4397-9dbb-9b2b20218538
3 files changed