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/fileutils.h b/webrtc/base/fileutils.h
index 8b09099..8f6ec52 100644
--- a/webrtc/base/fileutils.h
+++ b/webrtc/base/fileutils.h
@@ -85,7 +85,7 @@
   virtual DirectoryIterator* IterateDirectory();
 
   // Opens a file. Returns an open StreamInterface if function succeeds.
-  // Otherwise, returns NULL.
+  // Otherwise, returns null.
   // TODO: Add an error param to indicate failure reason, similar to
   // FileStream::Open
   virtual FileStream *OpenFile(const Pathname &filename,