Remove deprecated global variables in libevent that cause tsan errors.

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.
+   - Remove deprecated global variables, event_sigcb and event_gotsig
+     (essentially unused) that trigger tsan errors. (crbug/605894)
 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.