Define magic descriptors in one place.

content/common/zygote_commands_linux.h

 // 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.
 
 #ifndef CONTENT_COMMON_ZYGOTE_COMMANDS_LINUX_H_
 #define CONTENT_COMMON_ZYGOTE_COMMANDS_LINUX_H_
 
+#include "base/posix/global_descriptors.h"
+#include "ipc/ipc_descriptors.h"
+
 namespace content {
 
 // Contents of the initial message sent from the zygote to the browser when it
 // is ready to go.
 static const char kZygoteHelloMessage[] = "ZYGOTE_OK";
 
 // Maximum allowable length for messages sent to the zygote.
 const size_t kZygoteMaxMessageLength = 8192;
 
 // File descriptors initialized by the Zygote Host
-const int kZygoteSocketPairFd = 3;
-const int kZygoteRendererSocketFd = 5;
+const int kZygoteSocketPairFd =
+    kPrimaryIPCChannel + base::GlobalDescriptors::kBaseDescriptor;
 // This file descriptor is special. It is passed to the Zygote and a setuid
 // helper will be called to locate the process of the Zygote on the system.
 // This mechanism is used when multiple PID namespaces exist because of the
 // setuid sandbox.
 // It is very important that this file descriptor does not exist in multiple
 // processes.
 // This number must be kept in sync in sandbox/linux/suid/sandbox.c
 const int kZygoteIdFd = 7;
 
 // These are the command codes used on the wire between the browser and the
 // zygote.
 enum {
   // Fork off a new renderer.
   kZygoteCommandFork = 0,
 
   // Reap a renderer child.
   kZygoteCommandReap = 1,
 
   // Check what happened to a child process.
   kZygoteCommandGetTerminationStatus = 2,
 
   // Read a bitmask of kSandboxLinux*
   kZygoteCommandGetSandboxStatus = 3
 };
 
 }  // namespace content
 
 #endif  // CONTENT_COMMON_ZYGOTE_COMMANDS_LINUX_H_