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

webrtc/base/weak_ptr.cc

Index: webrtc/base/weak_ptr.cc
diff --git a/webrtc/base/weak_ptr.cc b/webrtc/base/weak_ptr.cc
index 4eef7eacc579e4fb6a75711f4c01587e43259730..8a07a87b7d0dcdd72c5c678bcc0c5f6cbd725938 100644
--- a/webrtc/base/weak_ptr.cc
+++ b/webrtc/base/weak_ptr.cc
@@ -73,7 +73,7 @@ void WeakReferenceOwner::Invalidate() {
   RTC_DCHECK(checker_.CalledSequentially());
   if (flag_.get()) {
     flag_->Invalidate();
-    flag_ = NULL;
+    flag_ = nullptr;
   }
 }