This is a clean copy of libevent-2.1.12-stable with the following modifications.

1. Add the following files:

- android/event2/event-config.h
- bsd/event2/event-config.h
- linux/event2/event-config.h
- mac/event2/event-config.h
- solaris/event2/event-config.h
- evconfig-private.h

These files are taken from libevent-2.1.12-stable built on the development
environment indicated by the first path component. You have to run
"./configure" and "make" to get all of the pre-processing done. The
event-config.h header can then be found in include/event2/ sub-directory and
evconfig-private.h can be found in the root source directory.

You then need to modify the EVENT__SIZEOF_* constants in the generated Linux,
Android, and BSD headers to be appropriate for both 32-bit and 64-bit platforms.
Mac doesn't need this since only 64-bit is supported. Use __LP64__ to
distinguish the two cases. If you get something wrong, the CHECK_EVENT_SIZEOF
static assertions in message_pump_libevent.cc will fail. If a new constant is
added, also add a static assertion for it to message_pump_libevent.cc.

You also need to modify the EVENT__HAVE_ARC4RANDOM and EVENT__HAVE_ARC4RANDOM_BUF
constants in the generated Linux header to account for the results of the arc4random
and arc4random_buf configure checks.

2. Apply the following patch from ipc/chromium/src/third_party/libevent/patches/:

- "prevent-use-of-evsig.patch".  Prevents using (via intentional crash) any
  of libevent's signal handling code; see bug 1616462.  This isn't strictly
  required for functionality but it's a local patch we'd like to retain.

Additionally, be careful to avoid clobbering changes to the various
event-config.h files which have been customized over time to avoid
various build bustages.
