Replace NULL with nullptr in all C++ files.

webrtc/pc/dtmfsender_unittest.cc

Index: webrtc/pc/dtmfsender_unittest.cc
diff --git a/webrtc/pc/dtmfsender_unittest.cc b/webrtc/pc/dtmfsender_unittest.cc
index 109760c3824a4a976adadfe62dc6a3d935ef9f36..698d00986e53d76292cd4b5100bc7421d95a0787 100644
--- a/webrtc/pc/dtmfsender_unittest.cc
+++ b/webrtc/pc/dtmfsender_unittest.cc
@@ -118,7 +118,7 @@ class FakeDtmfProvider : public DtmfProviderInterface {
 class DtmfSenderTest : public testing::Test {
  protected:
   DtmfSenderTest()
-      : track_(AudioTrack::Create(kTestAudioLabel, NULL)),
+      : track_(AudioTrack::Create(kTestAudioLabel, nullptr)),
         observer_(new rtc::RefCountedObject<FakeDtmfObserver>()),
         provider_(new FakeDtmfProvider()) {
     provider_->SetCanInsertDtmf(true);
@@ -283,7 +283,7 @@ TEST_F(DtmfSenderTest, InsertDtmfWhileSenderIsDeleted) {
   EXPECT_TRUE_SIMULATED_WAIT(observer_->tones().size() == 1, kMaxWaitMs,
                              fake_clock_);
   // Delete the sender.
-  dtmf_ = NULL;
+  dtmf_ = nullptr;
   // The queue should be discontinued so no more tone callbacks.
   SIMULATED_WAIT(false, 200, fake_clock_);
   EXPECT_EQ(1U, observer_->tones().size());