Bug 807168 - Move GCMarker into gc/Tracer.h; r=jonco

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extra : rebase_source : 031495e6bb5510009eb871866e9f5e3beda24211
Bug 807168 - Move rarely used code in GCMarker out of the header; r=jonco

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extra : rebase_source : f8895a2a2a00f31a45900cdbc718a19aed6262d9