Replace NULL with nullptr in all C++ files.

webrtc/pc/dtmfsender.cc

Index: webrtc/pc/dtmfsender.cc
diff --git a/webrtc/pc/dtmfsender.cc b/webrtc/pc/dtmfsender.cc
index bbb100ea61789d3b295170131793af258c8ec3cb..0de9637d02227995f670290e2b56328908ecfa3c 100644
--- a/webrtc/pc/dtmfsender.cc
+++ b/webrtc/pc/dtmfsender.cc
@@ -76,16 +76,16 @@ DtmfSender::DtmfSender(AudioTrackInterface* track,
                        rtc::Thread* signaling_thread,
                        DtmfProviderInterface* provider)
     : track_(track),
-      observer_(NULL),
+      observer_(nullptr),
       signaling_thread_(signaling_thread),
       provider_(provider),
       duration_(kDtmfDefaultDurationMs),
       inter_tone_gap_(kDtmfDefaultGapMs) {
-  RTC_DCHECK(signaling_thread_ != NULL);
+  RTC_DCHECK(signaling_thread_ != nullptr);
   // TODO(deadbeef): Once we can use shared_ptr and weak_ptr,
   // do that instead of relying on a "destroyed" signal.
   if (provider_) {
-    RTC_DCHECK(provider_->GetOnDestroyedSignal() != NULL);
+    RTC_DCHECK(provider_->GetOnDestroyedSignal() != nullptr);
     provider_->GetOnDestroyedSignal()->connect(
         this, &DtmfSender::OnProviderDestroyed);
   }
@@ -100,7 +100,7 @@ void DtmfSender::RegisterObserver(DtmfSenderObserverInterface* observer) {
 }
 
 void DtmfSender::UnregisterObserver() {
-  observer_ = NULL;
+  observer_ = nullptr;
 }
 
 bool DtmfSender::CanInsertDtmf() {
@@ -229,7 +229,7 @@ void DtmfSender::DoInsertDtmf() {
 void DtmfSender::OnProviderDestroyed() {
   LOG(LS_INFO) << "The Dtmf provider is deleted. Clear the sending queue.";
   StopSending();
-  provider_ = NULL;
+  provider_ = nullptr;
 }
 
 void DtmfSender::StopSending() {