Bug 828502 - let nsMainThreadPtrHandle::get() handle nullptrs; r=bholley, a=blocking-basecamp+
Bug 829525 - nsMainThreadPtrHolder needs a virtual destructor, r=dougt, a=blocking-basecamp+
Bug 827503 - use nsMainThreadPtrHolders when passing XPCWrappedJSes around off main thread, r=bholley,jst a=blocking-basecamp+