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

BUG=webrtc:7147

Review-Url: https://codereview.webrtc.org/2718663005
Cr-Commit-Position: refs/heads/master@{#16878}
diff --git a/webrtc/base/pathutils.cc b/webrtc/base/pathutils.cc
index 80df0ef..f79c298 100644
--- a/webrtc/base/pathutils.cc
+++ b/webrtc/base/pathutils.cc
@@ -43,7 +43,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 bool Pathname::IsFolderDelimiter(char ch) {
-  return (NULL != ::strchr(FOLDER_DELIMS, ch));
+  return (nullptr != ::strchr(FOLDER_DELIMS, ch));
 }
 
 char Pathname::DefaultFolderDelimiter() {