Open MessagePumpLibevent's pipe with O_CLOEXEC

This prevents the pipe's fds from being leaked into child processes.
This is a cherry-pick of https://crrev.com/80c77bfd86 .

TEST=git cl try
BUG=chromium:653930

Change-Id: I1d9ea26e41a23dc7ca5cd6d4b2133a2e69b1259c
Reviewed-on: https://chromium-review.googlesource.com/395446
Commit-Ready: Luis Hector Chavez <lhchavez@google.com>
Tested-by: Luis Hector Chavez <lhchavez@google.com>
Reviewed-by: Luis Hector Chavez <lhchavez@google.com>
3 files changed