Include Panasonic Smart TVs and other old browsers in positive DTS gate

[jeremies]

This PR will...

Prevent us from using negative DTS values in AVC moof atoms in Panasonic Smart TVs.

Why is this Pull Request needed?

Some new Panasonic Smart TVs have a navigator.userAgent string like this one:

HbbTV/1.4.1 (;Panasonic;VIERA 2020;a.393;4301-0003 0008-0000;) PanasonicSDK/2017

These Smart TVs use an old version of Chrome like Chrome 51, and this browser needs the positive DTS gate. It's like the following pull requests for Panasonic Smart TVs:

#3224
#2995

Are there any points in the code the reviewer needs to double check?

Resolves issues:

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

[OrenMe]

Suggest to also expose a config value so we won't need to patch version each time we discover an exotic user agent or other condition

[robwalch]

Maybe it makes sense to require positive DTS by default. Then the user agent check can be for allowing negative DTS specifically where mainstream browsers can take advantage of it? The reason for defaulting this was aimed at Chrome and reducing start gaps in modern web browsers, but it should not come at the cost of stability in older platforms that we failed to match in the UA check.

[robwalch]

To avoid another UA check how about reversing the gate?

    requiresPositiveDts = !(
      (!!chromeVersion && chromeVersion >= 75) ||
      (!!safariWebkitVersion && safariWebkitVersion >= 600)
    );

[jeremies]

It makes sense for me to reverse the gate. I'll push the reversed gate on this branch.