move libevent into base

third_party/libevent/README.chromium

-Name: libevent
-URL: http://libevent.org/
-Version: 1.4.15
-License: BSD
-Security Critical: yes
-
-Local Modifications:
-Rather than use libevent's own build system, we just build a Chrome
-static library using GYP.
-
-1) Run configure and "make event-config.h" on Linux, FreeBSD, Solaris,
-   and Mac and copy config.h and event-config.h to linux/, freebsd/,
-   solaris/, and mac/ respectively.
-2) Add libevent.gyp.
-3) chromium.patch is applied to make the following changes:
-   - Allow libevent to be used without being installed by changing <...>
-     #includes to "...".
-   - Fix a race condition in event_del.
-   - Optimistically assume CLOCK_MONOTONIC is available and fallback if it
-     fails, rather than explicitly testing for it.
-   - Remove an unneeded variable that causes a -Werror build failure.
-   - Add an #ifndef to fix a preprocessor redefined -Werror build failure.
-   - Revert the patch from http://sourceforge.net/p/levent/bugs/223/ that
-     introduces use-after-free memory corruption when an event callback frees
-     the struct event memory.
-4) The directories WIN32-Code and WIN32-Prj are not included.
-5) The configs for android were copied from Linux's which were very close to
-   android one with the exception of HAVE_FD_MASK and HAVE_STRLCPY.
-6) Add files to support building with the PNaCl toolchain. Added
-   libevent_nacl_nonsfi.gyp for build rule. nacl_nonsfi/config.h and
-   nacl_nonsfi/event-config.h are derived from linux/ counterparts.
-   nacl_nonsfi/random.c is also added to provide the random() function,
-   which is missing in the newlib-based PNaCl toolchain.
-7) Stub out signal.c for nacl_helper_nonsfi. socketpair() will be prohibited
-   by sandbox in nacl_helper_nonsfi.