Use Mojo pipes to signal sync IPC events

mojo/public/cpp/system/watcher.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_PUBLIC_CPP_SYSTEM_WATCHER_H_
 #define MOJO_PUBLIC_CPP_SYSTEM_WATCHER_H_
 
 #include <memory>
 
 #include "base/callback.h"
@@ skipping 59 matching lines @@
   //
   // Destroying the Watcher implicitly calls |Cancel()|.
   MojoResult Start(Handle handle,
                    MojoHandleSignals signals,
                    const ReadyCallback& callback);
 
   // Cancels the current watch. Once this returns, the callback previously
   // 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;
 
   void OnHandleReady(MojoResult result);
 
   static void CallOnHandleReady(uintptr_t context,
                                 MojoResult result,
                                 MojoHandleSignalsState signals_state,
                                 MojoWatchNotificationFlags flags);
@@ skipping 23 matching lines @@
   ReadyCallback callback_;
 
   base::WeakPtrFactory<Watcher> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(Watcher);
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_SYSTEM_WATCHER_H_