Implement a full shim for the VTTCue WebIDL.

Haven't implemented the exceptions thrown as we still need to
decide how to synthesize, or creat real, DOMExceptions.
Implement a full shim for the VTTRegion WebIDL.

Haven't added throwing exceptions yet as we still need to decide how to
synthesize, or throw real, DOMExceptions.
Properly handle VTTCue and VTTRegion to and from JSON.

- VTTCues and VTTRegions should be able to be constructed from JSON.
- NodeVTT should actually return VTTCue and VTTRegion objects, not just JSON
versions of them. VTTCue should be JSONified without _reset, getCueAsHTML,
and displayState properties.
Attach computedLinePos() to VTTCue's prototype.
Update processing model to reflect changes in the spec.

These include changes for the new line alignment and position
alignment settings.
Cue2json should write processing model tests to different JSON filename.
Add processing model tests.