Fix missing native HTML5 tracks

[mister-ben]

Description

Fixes missing text tracks when using native tracks in Chrome etc, fixes #2689
Tracks are currently added when the native text tracks addtrack event is triggered. Depending on how the player is initialised, this does not occur or fires before the event listener has been added.

Specific Changes proposed

• Adds tracks when the HTML5 tech is initialised.
• Modifies addTrack() to not add a track that already exists. This is to prevent duplication where addtrack is triggered.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[gkatsev]

Array#includes is ES6 only, we can't use it.

[gkatsev]

Also, could you rebase against stable? We could get this out as a patch update.

[mister-ben]

I've removed includes(), but rebasing against stable will add other commits as before. Should I do that?

[gkatsev]

Don't worry about it then, when merging I'll see if I can get it merged correctly into stable, otherwise, we'll just merge it into master and have it be part of the next minor release.

[gkatsev]

Ah, figured out how to do it locally. You'd have wanted to do an interactive rebase and then delete each of the commits that aren't part of this PR

$ git rebase -i stable

and only leave the last two commits (fe91dc8, and 5fb69e3). But I'm getting this merged locally along with a small tweak to make sure that forloop doesn't run if tt is null.

[gkatsev]

This is how it looks like (#3224), I created the PR so that I can run browserstack on it before merging.