Replace NULL with nullptr or null in webrtc/base/.

webrtc/base/weak_ptr.cc

 /*
  *  Copyright 2016 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 55 matching lines @@
   if (!HasRefs())
     flag_ = new RefCountedObject<WeakReference::Flag>();
 
   return WeakReference(flag_.get());
 }
 
 void WeakReferenceOwner::Invalidate() {
   RTC_DCHECK(checker_.CalledSequentially());
   if (flag_.get()) {
     flag_->Invalidate();
-    flag_ = NULL;
+    flag_ = nullptr;
   }
 }
 
 WeakPtrBase::WeakPtrBase() {}
 
 WeakPtrBase::~WeakPtrBase() {}
 
 WeakPtrBase::WeakPtrBase(const WeakReference& ref) : ref_(ref) {}
 
 }  // namespace internal
 }  // namespace rtc