This member variable is declared only in debug builds. This fixes the build
when this header is imported into non-unified builds.
This header is included in the build when xpcom/threads is built.
MozReview-Commit-ID: AF3YUqvVqET
--HG--
extra : rebase_source : f0065f0d326af069dc6c8a255fefbc0d2e2e9fe9