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

sandbox/linux/services/syscall_wrappers.h

Index: sandbox/linux/services/syscall_wrappers.h
diff --git a/sandbox/linux/services/syscall_wrappers.h b/sandbox/linux/services/syscall_wrappers.h
index 7f703e03b7bb6a2121d59ecdf4b5379d1086918c..2001f38f40b4f8d6dd1babc3e0bdca7cfc01f31e 100644
--- a/sandbox/linux/services/syscall_wrappers.h
+++ b/sandbox/linux/services/syscall_wrappers.h
@@ -33,6 +33,18 @@
                               pid_t* ctid,
                               decltype(nullptr) regs);
 
+// 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.
+SANDBOX_EXPORT pid_t
+ForkWithFlags(unsigned long flags, pid_t* ptid, pid_t* ctid);
+
 SANDBOX_EXPORT void sys_exit_group(int status);
 
 // The official system call takes |args| as void*  (in order to be extensible),