Replace NULL with nullptr in all C++ files.

webrtc/test/call_test.cc

Index: webrtc/test/call_test.cc
diff --git a/webrtc/test/call_test.cc b/webrtc/test/call_test.cc
index 833e53c4652288b3a268103432c07214876d08d5..b6456297d7edbce9839006623e0a737375310f64 100644
--- a/webrtc/test/call_test.cc
+++ b/webrtc/test/call_test.cc
@@ -146,12 +146,12 @@ void CallTest::Start() {
     audio_recv_stream->Start();
   for (FlexfecReceiveStream* flexfec_recv_stream : flexfec_receive_streams_)
     flexfec_recv_stream->Start();
-  if (frame_generator_capturer_.get() != NULL)
+  if (frame_generator_capturer_.get() != nullptr)
     frame_generator_capturer_->Start();
 }
 
 void CallTest::Stop() {
-  if (frame_generator_capturer_.get() != NULL)
+  if (frame_generator_capturer_.get() != nullptr)
     frame_generator_capturer_->Stop();
   for (FlexfecReceiveStream* flexfec_recv_stream : flexfec_receive_streams_)
     flexfec_recv_stream->Stop();