Bug 1579489 - Check the persistent permission checkbox by default for...

Bug 1579489 - Check the persistent permission checkbox by default for WebExtension WebRTC prompts. r=jib

This is done to offset the requirement that extensions have to get persistent permission before
they can start using WebRTC in popups.

Differential Revision: https://phabricator.services.mozilla.com/D45335

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extra : moz-landing-system : lando