Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_processing/aec/echo_cancellation_unittest.cc

Index: webrtc/modules/audio_processing/aec/echo_cancellation_unittest.cc
diff --git a/webrtc/modules/audio_processing/aec/echo_cancellation_unittest.cc b/webrtc/modules/audio_processing/aec/echo_cancellation_unittest.cc
index 20b63d667d5b56f8b692f03f3175dc735d94d498..9a1f8345754e9fff19b42436a01f0c7c713d67f8 100644
--- a/webrtc/modules/audio_processing/aec/echo_cancellation_unittest.cc
+++ b/webrtc/modules/audio_processing/aec/echo_cancellation_unittest.cc
@@ -31,9 +31,9 @@ TEST(EchoCancellationTest, CreateAndFreeHasExpectedBehavior) {
 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;