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

webrtc/base/signalthread.cc

Index: webrtc/base/signalthread.cc
diff --git a/webrtc/base/signalthread.cc b/webrtc/base/signalthread.cc
index 40df02f7b5cc9fd244c418130605b3cca8a78b33..be2741e7649fa99d0bc7b1503089322f6e55ab9d 100644
--- a/webrtc/base/signalthread.cc
+++ b/webrtc/base/signalthread.cc
@@ -113,7 +113,7 @@ void SignalThread::OnMessage(Message *msg) {
       // SignalThread this makes an assert in thread.cc fire.
       //
       // Calling Stop() on the worker ensures that the OS thread that underlies
-      // the worker will finish, and will be set to NULL, enabling us to call
+      // the worker will finish, and will be set to null, enabling us to call
       // Start() again.
       worker_.Stop();
       SignalWorkDone(this);
@@ -144,7 +144,7 @@ void SignalThread::Run() {
 
 void SignalThread::OnMainThreadDestroyed() {
   EnterExit ee(this);
-  main_ = NULL;
+  main_ = nullptr;
 }
 
 bool SignalThread::Worker::IsProcessingMessages() {