Bug 932466 - Fix two exact rooting hazards in dom; r=smaug

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extra : rebase_source : 8085170b6ab76912962c19dce9e409e2f6026058
Bug 932364 - Fix an exact rooting hazard around AddSizeOfTab; r=njn

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extra : rebase_source : cb0ab85f7f850ce3c250dd75ff32a416dba7b153