Replace NULL with nullptr in all C++ files.

webrtc/base/asyncinvoker.cc

Index: webrtc/base/asyncinvoker.cc
diff --git a/webrtc/base/asyncinvoker.cc b/webrtc/base/asyncinvoker.cc
index 83a873811e7258a16a982c39609598114ca78055..fd064cca67130e3a8a83199f375a8aaec75ab70b 100644
--- a/webrtc/base/asyncinvoker.cc
+++ b/webrtc/base/asyncinvoker.cc
@@ -30,7 +30,7 @@ void AsyncInvoker::OnMessage(Message* msg) {
       static_cast<ScopedRefMessageData<AsyncClosure>*>(msg->pdata);
   scoped_refptr<AsyncClosure> closure = data->data();
   delete msg->pdata;
-  msg->pdata = NULL;
+  msg->pdata = nullptr;
 
   // Execute the closure and trigger the return message if needed.
   closure->Execute();
@@ -132,8 +132,8 @@ 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_ == null means do not trigger the callback.
+  calling_thread_ = nullptr;
 }
 
 }  // namespace rtc