Bug 1058396 - Make String(aSymbol) return the symbol's description, as specified in ES6 draft rev 27. r=jorendorff.

--HG--
extra : rebase_source : 40b8293fe1fa14667d00f00170b69f3138586279
Bug 1057587 - Crash [@ JSString::ensureLinear] with Symbol and environment object.

--HG--
extra : rebase_source : af77bb098aefe1191c5ea26e5dfcf1d775fa3c84
extra : amend_source : 07dc0c27907e76748fcc34dc74e97fc914ab4ac7