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

webrtc/base/unixfilesystem.h

Index: webrtc/base/unixfilesystem.h
diff --git a/webrtc/base/unixfilesystem.h b/webrtc/base/unixfilesystem.h
index 8338e5cd0bb504c4f30d4183f91b25a72309a4bd..6f9095910193cc8f07e7cfb65e108fe5caed58bb 100644
--- a/webrtc/base/unixfilesystem.h
+++ b/webrtc/base/unixfilesystem.h
@@ -35,7 +35,7 @@ class UnixFilesystem : public FilesystemInterface {
 #endif
 
   // Opens a file. Returns an open StreamInterface if function succeeds.
-  // Otherwise, returns NULL.
+  // Otherwise, returns null.
   FileStream* OpenFile(const Pathname& filename,
                        const std::string& mode) override;