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/unittest_main.cc b/webrtc/base/unittest_main.cc
index 3de0528..1600217 100644
--- a/webrtc/base/unittest_main.cc
+++ b/webrtc/base/unittest_main.cc
@@ -66,7 +66,7 @@
   testing::InitGoogleTest(&argc, argv);
   rtc::FlagList::SetFlagsFromCommandLine(&argc, argv, false);
   if (FLAG_help) {
-    rtc::FlagList::Print(NULL, false);
+    rtc::FlagList::Print(nullptr, false);
     return 0;
   }