Store a map of eTLDs to script tasks in the constellation

[jdm]

This will allow us to avoid spawning a new script task per cross-origin iframe if the iframes share the same eTLD.

Blocks #5236.

[jdm]

This requires making decisions about what happens when navigating a browsing context to a new origin for which there is an existing script task. The current script task code only understands a single root page; if there's no relationship between new newly-navigated context and the existing root page, it could get ugly.

[asajeffrey]

Dupe of #633? I think we want eTLD+1, not eTLD.

[asajeffrey]

Closed by #14211.