Bug 1093307 - Part 2: Make OOM flushing paths more straightforward; r=jonco

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extra : rebase_source : 4634215f64ea96c2057f4ff609119b2a6cad008f
Bug 1074961 - Part 12: Simplify the locking in releaseArena; r=sfink

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extra : rebase_source : 3b1f8e728a31948a1bcb04aa3a46a5ea2b216aa4