Remove renderer notifications of memory pressure.

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 90 matching lines @@
   GuestViewMsgStart,
   // Note: CastCryptoMsgStart and CastChannelMsgStart reserved for Chromecast
   // internal code. Contact gunsch@ before changing/removing.
   CastCryptoMsgStart,
   CastChannelMsgStart,
   DataReductionProxyStart,
   ChromeAppBannerMsgStart,
   AttachmentBrokerMsgStart,
   RenderProcessMsgStart,
   PageLoadMetricsMsgStart,
-  MemoryMsgStart,
   IPCTestMsgStart,
   ArcInstanceMsgStart,
   ArcInstanceHostMsgStart,
   DistillerMsgStart,
   ArcCameraMsgStart,
   DWriteFontProxyMsgStart,
   MediaPlayerDelegateMsgStart,
   SurfaceViewManagerMsgStart,
   ExtensionWorkerMsgStart,
   SubresourceFilterMsgStart,
   LastIPCMsgStart  // Must come last.
 };
 
 #endif  // IPC_IPC_MESSAGE_START_H_