Move string files in base/ to the string subdirectory.

base/strings/string_util.h

Index: base/strings/string_util.h
diff --git a/base/string_util.h b/base/strings/string_util.h
similarity index 99%
copy from base/string_util.h
copy to base/strings/string_util.h
index 99ba091708445e179e49baa60a95ffa690e506b3..9e228bd4fd708bf026a2c3d2698ede5aeb95547a 100644
--- a/base/string_util.h
+++ b/base/strings/string_util.h
@@ -4,8 +4,8 @@
 //
 // This file defines utility functions for working with strings.
 
-#ifndef BASE_STRING_UTIL_H_
-#define BASE_STRING_UTIL_H_
+#ifndef BASE_STRINGS_STRING_UTIL_H_
+#define BASE_STRINGS_STRING_UTIL_H_
 
 #include <ctype.h>
 #include <stdarg.h>   // va_list
@@ -573,4 +573,4 @@ struct ToUnsigned<short> {
   typedef unsigned short Unsigned;
 };
 
-#endif  // BASE_STRING_UTIL_H_
+#endif  // BASE_STRINGS_STRING_UTIL_H_