Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_processing/transient/wpd_tree.cc

Index: webrtc/modules/audio_processing/transient/wpd_tree.cc
diff --git a/webrtc/modules/audio_processing/transient/wpd_tree.cc b/webrtc/modules/audio_processing/transient/wpd_tree.cc
index fece6863fc4dc67525aa84533dfc575a4772a8fa..432b05ff621514f23d547d7cff6fe4e215d35b14 100644
--- a/webrtc/modules/audio_processing/transient/wpd_tree.cc
+++ b/webrtc/modules/audio_processing/transient/wpd_tree.cc
@@ -66,7 +66,7 @@ WPDTree::~WPDTree() {}
 
 WPDNode* WPDTree::NodeAt(int level, int index) {
   if (level < 0 || level > levels_ || index < 0 || index >= 1 << level) {
-    return NULL;
+    return nullptr;
   }
 
   return nodes_[(1 << level) + index].get();