Add DataReductionProxy IPC to determine if the Data Reduction Proxy was used.

ipc/ipc_message_start.h

 // Copyright 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.
 
 #ifndef IPC_IPC_MESSAGE_START_H_
 #define IPC_IPC_MESSAGE_START_H_
 
 // Used by IPC_BEGIN_MESSAGES so that each message class starts from a unique
 // base.  Messages have unique IDs across channels in order for the IPC logging
 // code to figure out the message class from its ID.
@@ skipping 100 matching lines @@
   LayoutTestMsgStart,
   NetworkHintsMsgStart,
   BluetoothMsgStart,
   NavigatorConnectMsgStart,
   CastMediaMsgStart,
   AwMessagePortMsgStart,
   GuestViewMsgStart,
   // Note: CastCryptoMsgStart reserved for Chromecast internal code.
   // Contact gunsch@ before changing/removing.
   CastCryptoMsgStart,
+  DataReductionProxyStart,
   LastIPCMsgStart  // Must come last.
 };
 
 #endif  // IPC_IPC_MESSAGE_START_H_