Backout of 925db0605e39 due to bustage. r=me
Backout of 266694bb2f66 due to bustage. r=me
Bug 853536 - Never write to size-only decoders when we've already gotten the size. r=jrmuizel

--HG--
extra : rebase_source : 14e8cf30dc665eab10e04ef643513bcfad633ea2