Use Mojo pipes to signal sync IPC events

ipc/mojo_event.h

Index: ipc/mojo_event.h
diff --git a/ipc/mojo_event.h b/ipc/mojo_event.h
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+// 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 IPC_MOJO_EVENT_H_
+#define IPC_MOJO_EVENT_H_
+
+#include "base/macros.h"
+#include "base/synchronization/lock.h"
+#include "mojo/public/cpp/system/message_pipe.h"
+
+namespace IPC {
+
+// A MojoEvent is a simple wrapper around a Mojo message pipe which supports
+// common WaitableEvent-like methods of Signal() and Reset(). This class exists
+// to support the transition from legacy IPC to Mojo IPC and is not intended for
+// general use outside of src/ipc. Unlike base::WaitableEvent, all MojoEvents
+// must be manually reset.
+class MojoEvent {
+ public:
+  // Constructs a new MojoEvent that is initially not signaled.
+  MojoEvent();
+
+  ~MojoEvent();
+
+  // Gets a Handle that can be waited on for this MojoEvent. When the Event is
+  // signaled, this handle will have |MOJO_HANDLE_SIGNAL_READABLE| satisfied.
+  const mojo::Handle& GetHandle() const { return wait_handle_.get(); }
+
+  void Signal();
+  void Reset();
+
+ private:
+  mojo::ScopedMessagePipeHandle signal_handle_;
+  mojo::ScopedMessagePipeHandle wait_handle_;
+
+  base::Lock lock_;
+  bool is_signaled_ = false;
+
+  DISALLOW_COPY_AND_ASSIGN(MojoEvent);
+};
+
+}  // namespace IPC
+
+#endif  // IPC_MOJO_EVENT_H_