Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_coding/neteq/dtmf_tone_generator_unittest.cc

Index: webrtc/modules/audio_coding/neteq/dtmf_tone_generator_unittest.cc
diff --git a/webrtc/modules/audio_coding/neteq/dtmf_tone_generator_unittest.cc b/webrtc/modules/audio_coding/neteq/dtmf_tone_generator_unittest.cc
index 9eea103ed5a01586f6ecf7122874cbc6590aca49..f134f33051c883ad243ebd9e69774ccf70ddd85e 100644
--- a/webrtc/modules/audio_coding/neteq/dtmf_tone_generator_unittest.cc
+++ b/webrtc/modules/audio_coding/neteq/dtmf_tone_generator_unittest.cc
@@ -171,9 +171,9 @@ TEST(DtmfToneGenerator, TestErrors) {
   // Initialize with valid parameters.
   ASSERT_EQ(0, tone_gen.Init(fs, event, attenuation));
   EXPECT_TRUE(tone_gen.initialized());
-  // NULL pointer to destination.
+  // null pointer to destination.
   EXPECT_EQ(DtmfToneGenerator::kParameterError,
-            tone_gen.Generate(kNumSamples, NULL));
+            tone_gen.Generate(kNumSamples, nullptr));
 }
 
 }  // namespace webrtc