Bug 902481 - Make EventTargetChainItem pool thread agnostic, v2, r=jst

--HG--
extra : rebase_source : e1e9d9b87a29fd1d3ad6645f87a7075295c3c91d
Bug 887241, Use strings, not atoms, for worker events, r=khuey

--HG--
extra : rebase_source : 3e294b1bde05a127db0a9fa1c31c48e9a514e11d