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

webrtc/base/asyncinvoker-inl.h

Index: webrtc/base/asyncinvoker-inl.h
diff --git a/webrtc/base/asyncinvoker-inl.h b/webrtc/base/asyncinvoker-inl.h
index d172eee0a6bc32cdde3d9d51b1dae0ed9e1471f9..bce68d94d4799caa1030a97ec5bb0034f0f7b8c2 100644
--- a/webrtc/base/asyncinvoker-inl.h
+++ b/webrtc/base/asyncinvoker-inl.h
@@ -68,7 +68,7 @@ class NotifyingAsyncClosureBase : public AsyncClosure,
     CritScope cs(&crit_);
     callback_ = callback;
   }
-  bool CallbackCanceled() const { return calling_thread_ == NULL; }
+  bool CallbackCanceled() const { return calling_thread_ == nullptr; }
 
  private:
   Location callback_posted_from_;