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This patch adds logic to the fetch chromedriver scripts to now enable selectively choosing what channel (e.g. Stable & Canary in our case). Additionally, we now have the ability to pin specific major version in case of any platform/OS/hardware issues. By default if no pinned version is specified, the N-1 stable version is fetched as a backup, along with the latest stable version. This is due to instances where not all hardware machines update at the same time/day and this should prevent perma failures. Otherwise this `backup` also serves as a way to select a specific version. Differential Revision: https://phabricator.services.mozilla.com/D233063