LSBUtils closes a file descriptor twice, once with fclose and then again with
close. It also does this on a background thread, during startup, which means
it tends to race with main thread code which opens files.
This patch fixes that, and also removes a work-around for the issue in the
MemMapSnapshot code.
MozReview-Commit-ID: JdDHt9ayFEl
--HG--
extra : rebase_source : 2000ede41108d6312734d8df7db98272b33528fa
extra : amend_source : 1443a596fab3e3126a22d44787adaf5e12e4587c