gn: 20190403 -> 2020-03-09

This updates gn to the required version for chromiumDev (the recommended
version for the stable release of Chromium isn't sufficient [0]).

[0]: The Chromium build fails during the configuration phase:
ERROR at //mojo/public/tools/bindings/mojom.gni:393:16: Undefined identifier
               "cpp_typemaps",
               ^-------------