Move string files in base/ to the string subdirectory.

base/strings/stringprintf.cc

Index: base/strings/stringprintf.cc
diff --git a/base/stringprintf.cc b/base/strings/stringprintf.cc
similarity index 98%
rename from base/stringprintf.cc
rename to base/strings/stringprintf.cc
index 946ec726644a76e9b892ca5c1a28cbd55a0f8823..14d7733381ec8c81fbb278ff07361a928c7446ad 100644
--- a/base/stringprintf.cc
+++ b/base/strings/stringprintf.cc
@@ -2,12 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "base/stringprintf.h"
+#include "base/strings/stringprintf.h"
 
 #include <errno.h>
 
 #include "base/scoped_clear_errno.h"
-#include "base/string_util.h"
+#include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
 
 namespace base {