feat(WebVTT): Handle badly formed VTT

[dave-nicholas]

Handle remove chevrons that appear as part of the inner text of the element to avoid parse failure.

[avelad]

@gkatsev can your review it also? Thanks!

[gkatsev]

Yes, taking a look. Decided it was worth taking a look at the spec, and it seems like < isn't allowed in cue text body, but > is allowed: https://www.w3.org/TR/webvtt/#webvtt-cue-text-span

I've also confirmed the behavior in firefox, chrome, and safari, where if there's < things after it get swallowed up and not rendered.

For example:

t.addCue(new VTTCue(0, 10, '<c>foobar < > > fdjkfdj</c>'))

will only render foobar > fdjkfdj

and

t.addCue(new VTTCue(0, 10, '<c>foobar > foo >foo <test> > < fdjkfdj</c>')) 

will only render foobar > foo >foo >

and

t.addCue(new VTTCue(0, 10, '<c>foobar --> foobaz</c>'))

will render as foobar --> foobaz

Given this, we can probably even simplify the escape a bit.

[dave-nicholas]

I will remove the function I added to string utils and create a method in VTT parser just for this.