Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_processing/transient/transient_detector.cc

Index: webrtc/modules/audio_processing/transient/transient_detector.cc
diff --git a/webrtc/modules/audio_processing/transient/transient_detector.cc b/webrtc/modules/audio_processing/transient/transient_detector.cc
index 987ff8130bba0c49b464aeee5bd7222c9052a0c4..36e6ea7ccbb332d3a1f761bcd8169ea15b4b3cad 100644
--- a/webrtc/modules/audio_processing/transient/transient_detector.cc
+++ b/webrtc/modules/audio_processing/transient/transient_detector.cc
@@ -146,7 +146,7 @@ float TransientDetector::Detect(const float* data,
 // multiplied by the detection result to avoid false positives.
 float TransientDetector::ReferenceDetectionValue(const float* data,
                                                  size_t length) {
-  if (data == NULL) {
+  if (data == nullptr) {
     using_reference_ = false;
     return 1.f;
   }