Fix false positive DHECK in event_posix.cc

It's OK that state is kDown when a thread is woken after being
signalled; another tread might also have woken and already set the state
to kDown.

Even if this waking and aquiring lock has been delayed until other
threads have set state to kDown and then subsequently to kUp again with
a new Set() call, it's OK to just set kDown and return since this thread
is returning kEventSignaled after the (new) Set() has ben called.

R=pbos@webrtc.org
TBR=tommi@webrtc.org

BUG=

Review URL: https://webrtc-codereview.appspot.com/36109004

Cr-Commit-Position: refs/heads/master@{#8334}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8334 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/system_wrappers/source/event_posix.cc

@@ -151,7 +151,7 @@ EventTypeWrapper EventPosix::Wait(unsigned long timeout) {
   // Be careful to only change the state if we're about to report that the
   // event was signaled.
   if (ret_val == 0) {
-    DCHECK(state_ == kUp);
+    // state_ might already be kDown, in case of multiple waiters. That's OK.
     state_ = kDown;
   }