Mojo type validation breaks extension chrome.serial API

Revert the strict |instanceof Array| codec.js check so that Mojo can roll.

This is just a copy of dcheng's CL:
https://codereview.chromium.org/827203002/ so that someone from the Mojo
team can land it.

BUG=445512
R=jimbe@chromium.org

Review URL: https://codereview.chromium.org/827283002