Fix scoped_refptr construction from NULL

This is a precursor to adding a new scoped_refptr(std::nullptr_t)
constructor. The implicit conversion from NULL to scoped_refptr<T>
causes a compilation error with the new constructor. Replace NULL with
nullptr in any files where this is a problem.

This CL was uploaded by git cl split.

R=nharper@chromium.org

Bug: 1018887
Change-Id: Id2d23759b383b43fea02bfba279ec96286da5eb9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884997
Auto-Submit: kylechar <kylechar@chromium.org>
Reviewed-by: Nick Harper <nharper@chromium.org>
Commit-Queue: Nick Harper <nharper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710049}

net/test/cert_test_util_nss.cc

@@ -75,19 +75,19 @@ scoped_refptr<X509Certificate> ImportClientCertAndKeyFromFile(
     ScopedCERTCertificate* nss_cert) {
   if (!ImportSensitiveKeyFromFile(dir, key_filename, slot)) {
     LOG(ERROR) << "Could not import private key from file " << key_filename;
-    return NULL;
+    return nullptr;
   }
 
   scoped_refptr<X509Certificate> cert(ImportCertFromFile(dir, cert_filename));
 
   if (!cert.get()) {
     LOG(ERROR) << "Failed to parse cert from file " << cert_filename;
-    return NULL;
+    return nullptr;
   }
 
   *nss_cert = ImportClientCertToSlot(cert, slot);
   if (!*nss_cert)
-    return NULL;
+    return nullptr;
 
   // |cert| continues to point to the original X509Certificate before the
   // import to |slot|. However this should not make a difference as NSS handles