Bug 838955 (part 1) - Fix rooting hazards in jsstr.cpp identified by static analysis.  r=sfink.

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extra : rebase_source : cc45015a1840ccb1c3146e0b4ad81f2226dac985
Bug 837874: Check ObserverService before adding observers in MediaManager r=derf
Bug 834882: move InParallelSection() out of ForkJoinSlice and rename Initialize() to InitializeTLS() r=billm