Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_processing/agc/loudness_histogram_unittest.cc

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 32 matching lines @@
 
 void LoudnessHistogramTest::TestClean() {
   EXPECT_EQ(hist_->CurrentRms(), 7.59621091765857e-02);
   EXPECT_EQ(hist_->AudioContent(), 0);
   EXPECT_EQ(hist_->num_updates(), 0);
 }
 
 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);
     hist_.reset(LoudnessHistogram::Create(buffer_size));
   } else {
     hist_.reset(LoudnessHistogram::Create());
   }
   TestClean();
 
   InputOutput io;
@@ skipping 33 matching lines @@
                     .c_str());
 }
 
 TEST_F(LoudnessHistogramTest, InactiveCircularBuffer) {
   RunTest(false, test::ResourcePath(
                      "audio_processing/agc/agc_no_circular_buffer", "dat")
                      .c_str());
 }
 
 }  // namespace webrtc