Implement full shims for VTTCue and VTTRegion WebIDLs

A few things:

* I haven't implemented the parts of the spec where DOMExceptions are thrown. We either need to figure out how to synthesize one of these, or create real ones. I've seen in places people suggesting to perform an action that throws the correct exception you want... we probably don't want to do that.
* We'll have to get shims for Object.defineProperties. Talking to Popcorn Maker devs it seems like IE8 will only allow one of these shims on a DOM element, so we could make VTTCue and VTTRegions into divs which store the values if we really need to.