Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_processing/vad/standalone_vad_unittest.cc

Index: webrtc/modules/audio_processing/vad/standalone_vad_unittest.cc
diff --git a/webrtc/modules/audio_processing/vad/standalone_vad_unittest.cc b/webrtc/modules/audio_processing/vad/standalone_vad_unittest.cc
index 5ed39b8299dc125e3e042caf338146a35b6c0109..586ecdd644c10c9c91dfd38d545e75d51a82912c 100644
--- a/webrtc/modules/audio_processing/vad/standalone_vad_unittest.cc
+++ b/webrtc/modules/audio_processing/vad/standalone_vad_unittest.cc
@@ -67,11 +67,11 @@ TEST(StandaloneVadTest, ActivityDetection) {
   FILE* pcm_file =
       fopen(test::ResourcePath("audio_processing/agc/agc_audio", "pcm").c_str(),
             "rb");
-  ASSERT_TRUE(pcm_file != NULL);
+  ASSERT_TRUE(pcm_file != nullptr);
 
   FILE* reference_file = fopen(
       test::ResourcePath("audio_processing/agc/agc_vad", "dat").c_str(), "rb");
-  ASSERT_TRUE(reference_file != NULL);
+  ASSERT_TRUE(reference_file != nullptr);
 
   // Reference activities are prepared with 0 aggressiveness.
   ASSERT_EQ(0, vad->set_mode(0));