Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_processing/agc/loudness_histogram.cc

Index: webrtc/modules/audio_processing/agc/loudness_histogram.cc
diff --git a/webrtc/modules/audio_processing/agc/loudness_histogram.cc b/webrtc/modules/audio_processing/agc/loudness_histogram.cc
index 9112fbbe800de84c715c3bd52733d9555c2f6ce0..fd6942f4e304befd9408e1a4b0e56a8d1057cc53 100644
--- a/webrtc/modules/audio_processing/agc/loudness_histogram.cc
+++ b/webrtc/modules/audio_processing/agc/loudness_histogram.cc
@@ -174,7 +174,7 @@ LoudnessHistogram* LoudnessHistogram::Create() {
 
 LoudnessHistogram* LoudnessHistogram::Create(int window_size) {
   if (window_size < 0)
-    return NULL;
+    return nullptr;
   return new LoudnessHistogram(window_size);
 }