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

base/process/launch.h

Index: base/process/launch.h
diff --git a/base/process/launch.h b/base/process/launch.h
index 0450ddf021e0faa7936bf7118951d050e185251b..dda2e3f3ba6d283278f7a6f957b07d5e4892bbbf 100644
--- a/base/process/launch.h
+++ b/base/process/launch.h
@@ -114,7 +114,9 @@ struct BASE_EXPORT LaunchOptions {
   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