Replace NULL with nullptr in all C++ files.

webrtc/base/thread_checker_unittest.cc

Index: webrtc/base/thread_checker_unittest.cc
diff --git a/webrtc/base/thread_checker_unittest.cc b/webrtc/base/thread_checker_unittest.cc
index dee16535783493ede49e0a9d3653804ddf58cf58..e995b507e95a2519ce0b919a1ef5e4265e638892 100644
--- a/webrtc/base/thread_checker_unittest.cc
+++ b/webrtc/base/thread_checker_unittest.cc
@@ -54,7 +54,7 @@ class CallDoStuffOnThread : public Thread {
   explicit CallDoStuffOnThread(ThreadCheckerClass* thread_checker_class)
       : Thread(),
         thread_checker_class_(thread_checker_class) {
-    SetName("call_do_stuff_on_thread", NULL);
+    SetName("call_do_stuff_on_thread", nullptr);
   }
 
   void Run() override { thread_checker_class_->DoStuff(); }
@@ -78,7 +78,7 @@ class DeleteThreadCheckerClassOnThread : public Thread {
       ThreadCheckerClass* thread_checker_class)
       : Thread(),
         thread_checker_class_(thread_checker_class) {
-    SetName("delete_thread_checker_class_on_thread", NULL);
+    SetName("delete_thread_checker_class_on_thread", nullptr);
   }
 
   void Run() override { thread_checker_class_.reset(); }