Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_processing/transient/wpd_tree.cc

 /*
  *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 48 matching lines @@
                                                   high_pass_coefficients,
                                                   coefficients_length));
     }
   }
 }
 
 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();
 }
 
 int WPDTree::Update(const float* data, size_t data_length) {
   if (!data || data_length != data_length_) {
     return -1;
   }
 
@@ skipping 29 matching lines @@
       if (update_result != 0) {
         return -1;
       }
     }
   }
 
   return 0;
 }
 
 }  // namespace webrtc