Support generic synchronization against an IPC::Channel

mojo/common/event.mojom

Index: mojo/common/event.mojom
diff --git a/mojo/common/event.mojom b/mojo/common/event.mojom
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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.
+
+module mojo.common.mojom;
+
+// A generic interface for using a message pipe as a simple signaling mechanism.
+interface Event {
+  Signal();
+};