Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_processing/agc/loudness_histogram_unittest.cc

Index: webrtc/modules/audio_processing/agc/loudness_histogram_unittest.cc
diff --git a/webrtc/modules/audio_processing/agc/loudness_histogram_unittest.cc b/webrtc/modules/audio_processing/agc/loudness_histogram_unittest.cc
index 089c05a023407a0ec4d57d2e84e1f0fe9a39c712..65b1339bba3c3bfa451061df4479ccc5a8f25a54 100644
--- a/webrtc/modules/audio_processing/agc/loudness_histogram_unittest.cc
+++ b/webrtc/modules/audio_processing/agc/loudness_histogram_unittest.cc
@@ -50,7 +50,7 @@ void LoudnessHistogramTest::TestClean() {
 void LoudnessHistogramTest::RunTest(bool enable_circular_buff,
                                     const char* filename) {
   FILE* in_file = fopen(filename, "rb");
-  ASSERT_TRUE(in_file != NULL);
+  ASSERT_TRUE(in_file != nullptr);
   if (enable_circular_buff) {
     int buffer_size;
     EXPECT_EQ(fread(&buffer_size, sizeof(buffer_size), 1, in_file), 1u);