Bug 981280.  Disable webvtt support on 28 for now, pending resolution of some issues.  a=lsblakk
Bug 983344. r=sfink, a=dveditz
Port bug 936236.  r=shu on the original patch, port pretty much trivial modulo a rename or two, a=dveditz
Bug 982974 - Be paranoid about neutering ArrayBuffer objects.  r=sfink, r=jorendorff, a=dveditz
Merge beta to m-r. a=merge