Use the libc clone wrapper in sys_clone.

sandbox/linux/services/syscall_wrappers_unittest.cc

Index: sandbox/linux/services/syscall_wrappers_unittest.cc
diff --git a/sandbox/linux/services/syscall_wrappers_unittest.cc b/sandbox/linux/services/syscall_wrappers_unittest.cc
index 861dd5feb1c02aa7ece96186694f6b7b675a19f1..79ed63d68910ab0e64f134165a6bda9878856424 100644
--- a/sandbox/linux/services/syscall_wrappers_unittest.cc
+++ b/sandbox/linux/services/syscall_wrappers_unittest.cc
@@ -39,14 +39,18 @@ TEST(SyscallWrappers, CloneParentSettid) {
 }
 
 TEST(SyscallWrappers, CloneChildSettid) {
+  // Warm up the libc pid cache, if there is one.
+  ASSERT_EQ(sys_getpid(), getpid());
+
   pid_t ctid = 0;
   pid_t pid =
       sys_clone(CLONE_CHILD_SETTID | SIGCHLD, nullptr, nullptr, &ctid, nullptr);
 
   const int kSuccessExit = 0;
   if (0 == pid) {
-    // In child.
-    if (sys_getpid() == ctid)
+    // In child.  Check both the raw getpid syscall and the libc getpid wrapper
+    // (which may rely on a pid cache).
+    if (sys_getpid() == ctid && getpid() == ctid)
       _exit(kSuccessExit);
     _exit(1);
   }