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

webrtc/base/asyncinvoker.cc

Index: webrtc/base/asyncinvoker.cc
diff --git a/webrtc/base/asyncinvoker.cc b/webrtc/base/asyncinvoker.cc
index aa65499c960cc268fb658eccae54cbb49083544d..a143e25f19341bd2d3368c991aca8ab9c1e4d418 100644
--- a/webrtc/base/asyncinvoker.cc
+++ b/webrtc/base/asyncinvoker.cc
@@ -147,8 +147,8 @@ void NotifyingAsyncClosureBase::CancelCallback() {
   // destructor of the dying object here by waiting until the callback
   // is done triggering.
   CritScope cs(&crit_);
-  // calling_thread_ == NULL means do not trigger the callback.
-  calling_thread_ = NULL;
+  // calling_thread_ == nullptr means do not trigger the callback.
+  calling_thread_ = nullptr;
 }
 
 }  // namespace rtc