See the latest value of dispatchLoop

The field, dispatchLoop, is accessed in activate() method and the thread
where Watchdog is running. Then, different threads could access the field
and could miss the latest value.

Change-Id: Id218522f5b318fb3d05833ba57d908a1bf6d2949

@@ -68,7 +68,7 @@ public class CoreEventDispatcher extends DefaultEventSinkRegistry
     private static final Event KILL_PILL = new AbstractEvent(null, 0) {
     };
 
-    private DispatchLoop dispatchLoop;
+    private volatile DispatchLoop dispatchLoop;
     private long maxProcessMillis = DEFAULT_EXECUTE_MS;
 
     // Means to detect long-running sinks