Move ForkWithFlags from sandbox/ to base/ and plug it into LaunchProcess.

base/process/launch.h

 // Copyright 2013 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.
 
 // This file contains functions for launching subprocesses.
 
 #ifndef BASE_PROCESS_LAUNCH_H_
 #define BASE_PROCESS_LAUNCH_H_
 
 #include <string>
@@ skipping 96 matching lines @@
   // rlim_max.  This pointer is owned by the caller and must live through
   // the call to LaunchProcess().
   const std::vector<int>* maximize_rlimits;
 
   // If true, start the process in a new process group, instead of
   // inheriting the parent's process group.  The pgid of the child process
   // will be the same as its pid.
   bool new_process_group;
 
 #if defined(OS_LINUX)
-  // If non-zero, start the process using clone(), using flags as provided.
+  // If non-zero, start the process using clone(), using flags as provided. Note
+  // that it is not safe to start a process with non-zero clone_flags if
+  // multiple threads are running.

[jln (very slow on Chromium), 2015/01/06 00:22:31]

1. Explain that the clone flags may not include a signal handlers (and that it
will always be SIGCHLD.

2. This warning is not at the right place and is not correct: even with
clone_flags == 0, it's not safe to fork from a multithreaded environment. But
we're going through hoops to make that sure that LaunchProcess() is written with
care so that the fork() + execve() sequence is safe. I don't think a warning
belongs here.

[rickyz (no longer on Chrome), 2015/01/06 02:45:37]

Done.

   int clone_flags;
 
   // By default, child processes will have the PR_SET_NO_NEW_PRIVS bit set. If
   // true, then this bit will not be set in the new child process.
   bool allow_new_privs;
 #endif  // defined(OS_LINUX)
 
 #if defined(OS_CHROMEOS)
   // If non-negative, the specified file descriptor will be set as the launched
   // process' controlling terminal.
@@ skipping 135 matching lines @@
 void ReplaceBootstrapPort(const std::string& replacement_bootstrap_name);
 #endif  // defined(OS_MACOSX)
 
 // Creates a LaunchOptions object suitable for launching processes in a test
 // binary. This should not be called in production/released code.
 BASE_EXPORT LaunchOptions LaunchOptionsForTest();
 
 }  // namespace base
 
 #endif  // BASE_PROCESS_LAUNCH_H_