Replace NULL with nullptr or null in webrtc/base/.

webrtc/base/stringutils.h

Index: webrtc/base/stringutils.h
diff --git a/webrtc/base/stringutils.h b/webrtc/base/stringutils.h
index de256581d75707d932980ca749bc62be61c6ef87..4c241f02ded2e648ab4699d8b2cf5cc0600b5f73 100644
--- a/webrtc/base/stringutils.h
+++ b/webrtc/base/stringutils.h
@@ -136,8 +136,8 @@ struct Traits {
 // String utilities which work with char or wchar_t
 ///////////////////////////////////////////////////////////////////////////////
 
-template<class CTYPE>
-inline const CTYPE* nonnull(const CTYPE* str, const CTYPE* def_str = NULL) {
+template <class CTYPE>
+inline const CTYPE* nonnull(const CTYPE* str, const CTYPE* def_str = nullptr) {
   return str ? str : (def_str ? def_str : Traits<CTYPE>::empty_str());
 }