Enable compiling GN by default.

tools/gn/trace.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/trace.h"
 
 #include <algorithm>
 #include <map>
 #include <sstream>
 #include <vector>
@@ skipping 253 matching lines @@
         needs_comma = true;
       }
       out << "}";
     }
     out << "}";
   }
 
   out << "]}";
 
   std::string out_str = out.str();
-  file_util::WriteFile(file_name, out_str.data(), out_str.size());
+  file_util::WriteFile(file_name, out_str.data(),
+                       static_cast<int>(out_str.size()));
 }