Replaces check for processingEvents. r=danm, darin a=hyatt

xpcom/threads/plevent.c

@@ -486,6 +486,11 @@ PL_ProcessPendingEvents(PLEventQueue* self)
     
     
     PR_EnterMonitor(self->monitor);
+    
+    if (self->processingEvents) {
+      PR_ExitMonitor(self->monitor);
+      return;
+    }
     self->processingEvents = PR_TRUE;
 
     /* Only process the events that are already in the queue, and