Bug 875305 - Replace slow webvtt caption selectors with classes. r=bz

We set the class caption-box class on the anonymous div which
acts as the parent container for the captions overlayed on
the video frame.

caption-text isn't currently used, but will be applied to
child elements by when bug 833382 lands.
Bug 876653 - Add missing isNative() test, r=jandem.