Replace NULL with nullptr in all C++ files.

webrtc/modules/audio_coding/acm2/acm_send_test.cc

Index: webrtc/modules/audio_coding/acm2/acm_send_test.cc
diff --git a/webrtc/modules/audio_coding/acm2/acm_send_test.cc b/webrtc/modules/audio_coding/acm2/acm_send_test.cc
index 5e68afc4d117ab580549d7e2547dcbae34ffbbcd..5d24181db2311de4b796a2303b6cac95373594ba 100644
--- a/webrtc/modules/audio_coding/acm2/acm_send_test.cc
+++ b/webrtc/modules/audio_coding/acm2/acm_send_test.cc
@@ -80,7 +80,7 @@ std::unique_ptr<Packet> AcmSendTestOldApi::NextPacket() {
   if (filter_.test(static_cast<size_t>(payload_type_))) {
     // This payload type should be filtered out. Since the payload type is the
     // same throughout the whole test run, no packet at all will be delivered.
-    // We can just as well signal that the test is over by returning NULL.
+    // We can just as well signal that the test is over by returning null.
     return nullptr;
   }
   // Insert audio and process until one packet is produced.