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

base/process/process.h

Index: base/process/process.h
diff --git a/base/process/process.h b/base/process/process.h
index ad8f8eec888f2e7c9297a15d0d693f36af0c871e..4fff1162a6b61da51e3b15b3f75a481c906fe57a 100644
--- a/base/process/process.h
+++ b/base/process/process.h
@@ -112,6 +112,25 @@ class BASE_EXPORT Process {
 #endif
 };
 
+#if defined(OS_LINUX)
+// A wrapper for clone with fork-like behavior, meaning that it returns the
+// child's pid in the parent and 0 in the child. |flags|, |ptid|, and |ctid| are
+// as in the clone system call (the CLONE_VM flag is not supported).
+//
+// This function uses the libc clone wrapper (which updates libc's pid cache)
+// internally, so callers may expect things like getpid() to work correctly
+// after in both the child and parent. An exception is when this code is run
+// under Valgrind. Valgrind does not support the libc clone wrapper, so the libc
+// pid cache may be incorrect after this function is called under Valgrind.
+//
+// As with fork(), callers should be extremely careful when calling this while
+// multiple threads are running, since at the time the fork happened, the
+// threads could have been in any state (potentially holding locks, etc.).
+// Callers should most likely call execve() in the child soon after calling
+// this.
+BASE_EXPORT pid_t ForkWithFlags(unsigned long flags, pid_t* ptid, pid_t* ctid);
+#endif
+
 }  // namespace base
 
 #endif  // BASE_PROCESS_PROCESS_PROCESS_H_