Bug 914521 - Hold a stack reference to mScriptGlobal when dispatching sync events. r=bz

Note also MMAdeathGrip earlier in the function.
Bug 915482 (part 1) - Move most of gc/Barrier-inl.h into gc/Barrier.h.  r=terrence.

--HG--
extra : rebase_source : e3fbf54420858cd5ae5328f4cf5c5001d1d0ccb9
Bug 911786 - Part 1: Store computed relative position offsets. r=dholbert
Bug 911786 - Part 2: Reapply relative positioning when moving frames without reflowing them. r=dholbert