Use Mojo pipes to signal sync IPC events

mojo/public/cpp/system/watcher.h

Index: mojo/public/cpp/system/watcher.h
diff --git a/mojo/public/cpp/system/watcher.h b/mojo/public/cpp/system/watcher.h
index 34a61f8b0c822d5be07a9c1a29550c264f9d3938..82f3e816fb22e94bb24cf05f7c7899195c38bdd7 100644
--- a/mojo/public/cpp/system/watcher.h
+++ b/mojo/public/cpp/system/watcher.h
@@ -77,6 +77,9 @@ class Watcher {
   // passed to |Start()| will never be called again for this Watcher.
   void Cancel();
 
+  Handle handle() const { return handle_; }
+  ReadyCallback ready_callback() const { return callback_; }
+
  private:
   class MessageLoopObserver;
   friend class MessageLoopObserver;