Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_processing/config_unittest.cc

Index: webrtc/modules/audio_processing/config_unittest.cc
diff --git a/webrtc/modules/audio_processing/config_unittest.cc b/webrtc/modules/audio_processing/config_unittest.cc
index 33f4ed152c48a882b0a1ef7ae8dcea7aa61842c8..1290b81cd87af9268d69ecc58f5ff33af1e3e101 100644
--- a/webrtc/modules/audio_processing/config_unittest.cc
+++ b/webrtc/modules/audio_processing/config_unittest.cc
@@ -50,7 +50,7 @@ TEST(Config, ReturnOptionWhenSet) {
 TEST(Config, SetNullSetsTheOptionBackToDefault) {
   Config config;
   config.Set<MyExperiment>(new MyExperiment(5, 1));
-  config.Set<MyExperiment>(NULL);
+  config.Set<MyExperiment>(nullptr);
   const MyExperiment& my_exp = config.Get<MyExperiment>();
   EXPECT_EQ(MyExperiment::kDefaultFactor, my_exp.factor);
   EXPECT_EQ(MyExperiment::kDefaultOffset, my_exp.offset);