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 2001f38f40b4f8d6dd1babc3e0bdca7cfc01f31e..7f703e03b7bb6a2121d59ecdf4b5379d1086918c 100644
--- a/sandbox/linux/services/syscall_wrappers.h
+++ b/sandbox/linux/services/syscall_wrappers.h
@@ -33,18 +33,6 @@ SANDBOX_EXPORT long sys_clone(unsigned long flags,
                               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),