Bug 865407 - Part 8: Update vtt.js to latest version. r=rillian

This completes the initial implementation of the processing algorithm.
The algorithm doesn't support vertical text yet, but that's not a
big issue since Firefox doesn't support it.
Bug 865407 - Part 9: Remove border-box and padding CSS on the caption overlay. r=rillian

This wasn't working correctly. Vtt.js now takes care of applying padding
to the video overlay.
Bug 969087 - Add GitHubUserContent.com to PSL. r=gerv
Bug 970614 - Remove code wrapped in #if 0 ... #endif blocks in PSM. r=keeler
Bug 974097 - AppendChild operation in LayersMessages.idplh should be called PrependChild. r=gal
Bug 974272 - Unbreak skia/qt on tier3 as well. r=gw280
Bug 974687 - Part 1: Add about 300 MOZ_OVERRIDE in layout/. r=dholbert
Bug 974687 - Part 2: Fix more than 80cols issues caused by MOZ_OVERRIDE annotation. r=dholbert
Bug 975412 - Fix length properties of Map and Set constructors. r=jorendorff