"Operation is insecure" in Nightly

[jscher2000]

Quick-and-dirty patch on Bugzilla here --

https://bugzilla.mozilla.org/show_bug.cgi?id=1585588#c6

-        const parsedDom = (new DOMParser()).parseFromString(
+        const parsedDom = (new iframe.contentWindow.DOMParser()).parseFromString(

-- likely should go here:

https://github.com/mozilla-services/screenshots/blob/master/webextension/onboarding/slides.js#L43

Related discussion:

https://www.reddit.com/r/firefox/comments/ddjol2/firefox_screenshots_went_haywire/