Bug 1046210 - Lift sandbox disablement checks out of async signal context. r=kang a=2.0+

The use of async-signal-unsafe logging in the DMD case was causing
occasional deadlocks in B2G stress testing -- unlike the other
async-signal-safety violations in this file, this one occurs when we
aren't already about to crash.

--HG--
extra : rebase_source : 77cef7590836427a598db7baa6dcad3356dc8e7d
extra : amend_source : 51377d4b9b28143038c420dee3f8a2a3f3b4f46e