For Linux, create a ZygoteForkDelegate that uses the nacl_helper executable

chrome/common/nacl_helper_linux.h

Index: chrome/common/nacl_helper_linux.h
diff --git a/chrome/common/nacl_helper_linux.h b/chrome/common/nacl_helper_linux.h
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+// 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.
+
+#ifndef CHROME_COMMON_NACL_HELPER_LINUX_H_
+#define CHROME_COMMON_NACL_HELPER_LINUX_H_
+#pragma once
+
+// A mini-zygote specifically for Native Client. This file defines
+// constants used to implement communication between the nacl_helper
+// process and the Chrome zygote.
+
+// Used by Helper to tell Zygote it has started successfully.
+#define kNaClHelperStartupAck "NACLHELPER_OK"
+// Used by Zygote to ask Helper to fork a new NaCl loader.
+#define kNaClForkRequest "NACLFORK"
+
+// The next set of constants define global Linux file descriptors.
+// For communications between NaCl loader and browser.
+// See also content/common/zygote_main_linux.cc and
+// http://code.google.com/p/chromium/wiki/LinuxZygote
+#define kNaClBrowserDescriptor 3
+// For communications between NaCl loader and zygote.
+// We put the kNaClZygoteDescriptor on 3 together with the
+// kNaClBrowserDescriptor. They are never used at the same
+// time, and this prevents /dev/urandom from using the fd.
+#define kNaClZygoteDescriptor 3
+// For communications between the NaCl loader process and
+// the SUID sandbox.
+#define kNaClSandboxDescriptor 5
+// NOTE: kNaClBrowserDescriptor and kNaClSandboxDescriptor must match
+// content/browser/zygote_main_linux.cc kBrowserDescriptor and
+// kMagicSandboxIPCDescriptor.
+
+// A fork request from the Zygote to the helper includes an array
+// of three file descriptors. These constants are used as indicies
+// into the array.
+// Used to pass in the descriptor for talking to the Browser
+#define kNaClBrowserFDIndex 0
+// The next two are used in the protocol for discovering the
+// child processes real PID from within the SUID sandbox. See
+// http://code.google.com/p/chromium/wiki/LinuxZygote
+#define kNaClDummyFDIndex 1
+#define kNaClParentFDIndex 2
+
+#endif  // CHROME_COMMON_NACL_HELPER_LINUX_H_