Move string files in base/ to the string subdirectory.

base/strings/string16.h

Index: base/strings/string16.h
diff --git a/base/string16.h b/base/strings/string16.h
similarity index 98%
copy from base/string16.h
copy to base/strings/string16.h
index f7a35d0a90282d770862720dc36f72203bf03c41..4fa412e9944ca79b4cfdf81aadebc999d8b79dd0 100644
--- a/base/string16.h
+++ b/base/strings/string16.h
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef BASE_STRING16_H_
-#define BASE_STRING16_H_
+#ifndef BASE_STRINGS_STRING16_H_
+#define BASE_STRINGS_STRING16_H_
 
 // WHAT:
 // A version of std::basic_string that provides 2-byte characters even when
@@ -186,4 +186,4 @@ class BASE_EXPORT std::basic_string<base::char16, base::string16_char_traits>;
 using base::char16;
 using base::string16;
 
-#endif  // BASE_STRING16_H_
+#endif  // BASE_STRINGS_STRING16_H_