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/unixfilesystem.h b/webrtc/base/unixfilesystem.h
index 8338e5c..6f90959 100644
--- a/webrtc/base/unixfilesystem.h
+++ b/webrtc/base/unixfilesystem.h
@@ -35,7 +35,7 @@
 #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;