UI: Seek bar doesn't work on iOS

[ismena]

Reproes on the nightly build

[joeyparrish]

This is a bit more subtle than I realized. The seek bar works on iOS, but it's much pickier. You have to click on the "ball" part of the slider, hold it, then drag.

We should find some way to fix this so that it works by touch event anywhere on the "bar" part of the slider.

[matvp91]

I've noticed this too, iOS requires you to grab the handle. Wouldn't it work with a custom touch(start/move/end) implementation in shaka.ui.RangeElement? If you want, I can have a play with this and support a PR.

[ismena]

@matvp91 We'd love a PR, thank you :)