Port get_gn_arg_value() to native Python code so that it works on Windows.
Port get_gn_arg_value() to native Python code so that it works on Windows.

This allows Goma to work on Win builder.

R=sky@chromium.org

Review URL: https://codereview.chromium.org/698853003

collinjackson/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,jianglu/mojo,chinmaygarde/mojo,chinmaygarde/mojo,collinjackson/mojo,afandria/mojo,afandria/mojo,afandria/mojo,chinmaygarde/mojo,collinjackson/mojo,afandria/mojo,chinmaygarde/mojo,afandria/mojo,chinmaygarde/mojo,collinjackson/mojo,jianglu/mojo,collinjackson/mojo,collinjackson/mojo,chinmaygarde/mojo,collinjackson/mojo,jianglu/mojo,afandria/mojo,collinjackson/mojo,jianglu/mojo,chinmaygarde/mojo,afandria/mojo,chinmaygarde/mojo,jianglu/mojo,chinmaygarde/mojo,collinjackson/mojo,jianglu/mojo