Implement ChannelMultiplexer.

remoting/proto/mux.proto

Index: remoting/proto/mux.proto
diff --git a/remoting/proto/mux.proto b/remoting/proto/mux.proto
new file mode 100644
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+++ b/remoting/proto/mux.proto
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+// Copyright (c) 2012 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.
+//
+// Protocol for the mux channel that multiplexes multiple channels.
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+
+package remoting.protocol;
+
+message MultiplexPacket {
+  // Channel ID. Each peer choses this value when it sends first packet to
+  // the other peer. It unique identified channel this packet belongs to.
+  // Channel ID is direction-specific, i.e. each channel has two IDs
+  // assigned to it: one for receiving and one for sending.
+  optional int32 channel_id = 1;
+
+  // Channel name. The name is used to identify channels before channel ID
+  // is assigned in the first message. This value must be included only
+  // in the first packet for a given channel. All other packets must be
+  // identified using channel ID.
+  optional string channel_name = 2;
+
+  optional bytes data = 3;
+}