Replace NULL with nullptr in all C++ files.

webrtc/modules/remote_bitrate_estimator/test/bwe.cc

Index: webrtc/modules/remote_bitrate_estimator/test/bwe.cc
diff --git a/webrtc/modules/remote_bitrate_estimator/test/bwe.cc b/webrtc/modules/remote_bitrate_estimator/test/bwe.cc
index 086e3c3264228e2089c4d0ee6795cc01d00c22ef..44829b8cb08cc49f7223df640c6f2afa34e2df51 100644
--- a/webrtc/modules/remote_bitrate_estimator/test/bwe.cc
+++ b/webrtc/modules/remote_bitrate_estimator/test/bwe.cc
@@ -99,7 +99,7 @@ BweSender* CreateBweSender(BandwidthEstimatorType estimator,
       return new NullBweSender();
   }
   assert(false);
-  return NULL;
+  return nullptr;
 }
 
 BweReceiver* CreateBweReceiver(BandwidthEstimatorType type,
@@ -118,7 +118,7 @@ BweReceiver* CreateBweReceiver(BandwidthEstimatorType type,
       return new BweReceiver(flow_id);
   }
   assert(false);
-  return NULL;
+  return nullptr;
 }
 
 // Take into account all LinkedSet content.