Move the common child process messages into their own file.

content/common/child_process_messages.h

+// Copyright (c) 2011 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.
+
+// Common IPC messages used for child processes.
+// Multiply-included message file, hence no include guard.
+
+#include "ipc/ipc_message_macros.h"
+
+#define IPC_MESSAGE_START ChildProcessMsgStart
+
+// Messages sent from the browser to the child process.
+
+// Tells the child process it should stop.
+IPC_MESSAGE_CONTROL0(ChildProcessMsg_AskBeforeShutdown)
+
+// Sent in response to ChildProcessHostMsg_ShutdownRequest to tell the child
+// process that it's safe to shutdown.
+IPC_MESSAGE_CONTROL0(ChildProcessMsg_Shutdown)
+
+#if defined(IPC_MESSAGE_LOG_ENABLED)
+// Tell the child process to begin or end IPC message logging.
+// Like above, this is used by all ChildProcesses.
+IPC_MESSAGE_CONTROL1(ChildProcessMsg_SetIPCLoggingEnabled,
+                     bool /* on or off */)
+#endif
+
+// Messages sent from the child process to the browser.
+
+IPC_MESSAGE_CONTROL0(ChildProcessHostMsg_ShutdownRequest)