Move nullable_string16.h to the string subdirectory.

base/strings/nullable_string16.h

Index: base/strings/nullable_string16.h
diff --git a/base/nullable_string16.h b/base/strings/nullable_string16.h
similarity index 72%
copy from base/nullable_string16.h
copy to base/strings/nullable_string16.h
index dbcd37c8470ad74758e4d216e07a34b618f05993..73fe81e0842034eba4d918cafeb58dcd8d22f2a0 100644
--- a/base/nullable_string16.h
+++ b/base/strings/nullable_string16.h
@@ -2,11 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef BASE_NULLABLE_STRING16_H_
-#define BASE_NULLABLE_STRING16_H_
+#ifndef BASE_STRINGS_NULLABLE_STRING16_H_
+#define BASE_STRINGS_NULLABLE_STRING16_H_
 
 #include "base/strings/string16.h"
 
+namespace base {
+
 // This class is a simple wrapper for string16 which also contains a null
 // state.  This should be used only where the difference between null and
 // empty is meaningful.
@@ -26,4 +28,10 @@ class NullableString16 {
   bool is_null_;
 };
 
-#endif  // BASE_NULLABLE_STRING16_H_
+}  // namespace
+
+// TODO(avi) update users of NullableString16 to use the namespace and remove
+// this "using".
+using base::NullableString16;
+
+#endif  // BASE_STRINGS_NULLABLE_STRING16_H_