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

BUG=webrtc:7147

Review-Url: https://codereview.webrtc.org/2718663005
Cr-Commit-Position: refs/heads/master@{#16878}
diff --git a/webrtc/base/weak_ptr.cc b/webrtc/base/weak_ptr.cc
index 4eef7ea..8a07a87 100644
--- a/webrtc/base/weak_ptr.cc
+++ b/webrtc/base/weak_ptr.cc
@@ -73,7 +73,7 @@
   RTC_DCHECK(checker_.CalledSequentially());
   if (flag_.get()) {
     flag_->Invalidate();
-    flag_ = NULL;
+    flag_ = nullptr;
   }
 }