Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_processing/aec/echo_cancellation_unittest.cc

 /*
  *  Copyright (c) 2013 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 13 matching lines @@
 TEST(EchoCancellationTest, CreateAndFreeHasExpectedBehavior) {
   void* handle = WebRtcAec_Create();
   ASSERT_TRUE(handle);
   WebRtcAec_Free(nullptr);
   WebRtcAec_Free(handle);
 }
 
 TEST(EchoCancellationTest, ApplyAecCoreHandle) {
   void* handle = WebRtcAec_Create();
   ASSERT_TRUE(handle);
-  EXPECT_TRUE(WebRtcAec_aec_core(NULL) == NULL);
+  EXPECT_TRUE(WebRtcAec_aec_core(nullptr) == nullptr);
   AecCore* aec_core = WebRtcAec_aec_core(handle);
-  EXPECT_TRUE(aec_core != NULL);
+  EXPECT_TRUE(aec_core != nullptr);
   // A simple test to verify that we can set and get a value from the lower
   // level |aec_core| handle.
   int delay = 111;
   WebRtcAec_SetSystemDelay(aec_core, delay);
   EXPECT_EQ(delay, WebRtcAec_system_delay(aec_core));
   WebRtcAec_Free(handle);
 }
 
 }  // namespace webrtc