Add CHECK to EventWrapper to see if there's a subtle bug there or not.

R=pbos@webrtc.org
BUG=

Review URL: https://webrtc-codereview.appspot.com/38039005

Cr-Commit-Position: refs/heads/master@{#8302}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8302 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/system_wrappers/source/event_posix.cc

@@ -18,6 +18,8 @@
 #include <sys/time.h>
 #include <unistd.h>
 
+#include "webrtc/base/checks.h"
+
 namespace webrtc {
 
 const long int E6 = 1000000;
@@ -146,6 +148,9 @@ EventTypeWrapper EventPosix::Wait(unsigned long timeout) {
     }
   }
 
+  if (ret_val == 0)
+    CHECK(state_ == kUp);
+
   state_ = kDown;
   pthread_mutex_unlock(&mutex_);