Fix Windows cast warning for GN unit test.

tools/gn/command_args.cc

Index: tools/gn/command_args.cc
diff --git a/tools/gn/command_args.cc b/tools/gn/command_args.cc
index 65edc20a9d2abecaa6001c0636d4b1fbb3d93e9a..6ee05f7229499a5bc9b593a5577c69f813343d02 100644
--- a/tools/gn/command_args.cc
+++ b/tools/gn/command_args.cc
@@ -254,7 +254,7 @@ int EditArgsFile(const std::string& build_dir) {
 #endif
       base::CreateDirectory(arg_file.DirName());
       base::WriteFile(arg_file, argfile_default_contents.c_str(),
-                      argfile_default_contents.size());
+                      static_cast<int>(argfile_default_contents.size()));
     }
 
     ScopedTrace editor_trace(TraceItem::TRACE_SETUP, "Waiting for editor");