Bug 1593170. Make the reftest-content.js functions FlushRendering and SynchronizeForSnapshot work on Fission child oop iframes. r=mattwoodrow,kmag

This changes them to return a promise that resolves when the work is done, but we still need to change the callers to handle this new return type and do the right thing when these functions do their work async-ly.

To do this we add a JSWindowActor called ReftestFission. reftest-content.js communicates with this actor via reftest.jsm.

Differential Revision: https://phabricator.services.mozilla.com/D51343

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extra : moz-landing-system : lando