Replace NULL with nullptr in all C++ files.

webrtc/video/video_send_stream.cc

Index: webrtc/video/video_send_stream.cc
diff --git a/webrtc/video/video_send_stream.cc b/webrtc/video/video_send_stream.cc
index 466b5c2f00ad0f7adb7ff6947456d9ef5632431d..5d5446dbda946f3fe047a944cc52501cdef24be6 100644
--- a/webrtc/video/video_send_stream.cc
+++ b/webrtc/video/video_send_stream.cc
@@ -140,7 +140,7 @@ VideoSendStream::Config::EncoderSettings::ToString() const {
   std::stringstream ss;
   ss << "{payload_name: " << payload_name;
   ss << ", payload_type: " << payload_type;
-  ss << ", encoder: " << (encoder ? "(VideoEncoder)" : "nullptr");
+  ss << ", encoder: " << (encoder ? "(VideoEncoder)" : "null");
   ss << '}';
   return ss.str();
 }
@@ -206,9 +206,9 @@ std::string VideoSendStream::Config::ToString() const {
   ss << "{encoder_settings: " << encoder_settings.ToString();
   ss << ", rtp: " << rtp.ToString();
   ss << ", pre_encode_callback: "
-     << (pre_encode_callback ? "(VideoSinkInterface)" : "nullptr");
+     << (pre_encode_callback ? "(VideoSinkInterface)" : "null");
   ss << ", post_encode_callback: "
-     << (post_encode_callback ? "(EncodedFrameObserver)" : "nullptr");
+     << (post_encode_callback ? "(EncodedFrameObserver)" : "null");
   ss << ", render_delay_ms: " << render_delay_ms;
   ss << ", target_delay_ms: " << target_delay_ms;
   ss << ", suspend_below_min_bitrate: " << (suspend_below_min_bitrate ? "on"