run iwyu on base!

base/string_split.cc

Index: base/string_split.cc
diff --git a/base/string_split.cc b/base/string_split.cc
index 44b5d067615c8d4cd1b1112d8cc2642a6ae76d6b..17e75a395016d244b1468ab806d0e9b51201d18d 100644
--- a/base/string_split.cc
+++ b/base/string_split.cc
@@ -2,12 +2,15 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "base/string_split.h"
+#include <stddef.h>
+#include <string>
+#include <utility>
+#include <vector>
 
 #include "base/logging.h"
+#include "base/string16.h"
 #include "base/string_util.h"
 #include "base/third_party/icu/icu_utf.h"
-#include "base/utf_string_conversions.h"
 
 namespace base {