add polyFillFindIndex function in order to bug fix in IE11

[nicosang]

Hi,

this PR has to solve issue #2489.

Nico

[davemevans]

This solution is ok, but I think it would be preferable to rework the section calling findIndex rather than modify the Array prototype. I think something like the below (NOTE: UNTESTED) should have the same effect and will transpile correctly with our current setup:

tracks.some((item, idx) => {
    if (item.lang === defaultLanguage) {
         this.setTextTrack(idx);
         index = idx;
         return true;
    }
});

[nicosang]

you're right @bbcrddave ....it's a little bit simpliest.... ;-)

Many thanks!