Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_processing/aec/system_delay_unittest.cc

Index: webrtc/modules/audio_processing/aec/system_delay_unittest.cc
diff --git a/webrtc/modules/audio_processing/aec/system_delay_unittest.cc b/webrtc/modules/audio_processing/aec/system_delay_unittest.cc
index 141d608008cb1c875c78882f39a8bafd84ee6b0f..7d6f78c717cd47b6cb21b95b5cb7e89e1dcc2628 100644
--- a/webrtc/modules/audio_processing/aec/system_delay_unittest.cc
+++ b/webrtc/modules/audio_processing/aec/system_delay_unittest.cc
@@ -51,7 +51,7 @@ class SystemDelayTest : public ::testing::Test {
 };
 
 SystemDelayTest::SystemDelayTest()
-    : handle_(NULL), self_(NULL), samples_per_frame_(0) {
+    : handle_(nullptr), self_(nullptr), samples_per_frame_(0) {
   // Dummy input data are set with more or less arbitrary non-zero values.
   for (int i = 0; i < kSamplesPerChunk; i++) {
     far_[i] = 257.0;
@@ -71,7 +71,7 @@ void SystemDelayTest::SetUp() {
 void SystemDelayTest::TearDown() {
   // Free AEC
   WebRtcAec_Free(handle_);
-  handle_ = NULL;
+  handle_ = nullptr;
 }
 
 // In SWB mode nothing is added to the buffer handling with respect to