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/messagedigest.cc b/webrtc/base/messagedigest.cc
index 5e8621c..05c8c4f 100644
--- a/webrtc/base/messagedigest.cc
+++ b/webrtc/base/messagedigest.cc
@@ -34,7 +34,7 @@
   MessageDigest* digest = new OpenSSLDigest(alg);
   if (digest->Size() == 0) {  // invalid algorithm
     delete digest;
-    digest = NULL;
+    digest = nullptr;
   }
   return digest;
 }