Add references to cross-origin script tasks to the page tree

[jdm]

When we have a cross-origin iframe, the page tree should contain a reference to its event loop, along with uniquely-identifying features. This will allow the implementation of APIs like postMessage, and set the stage for the implementation of #3939.

Blocks #5236.

[KiChjang]

Does this mean that a page should have ways of getting child script tasks?

[jdm]

Basically, yes. This ties in a bit with the changes in #8658 which add a concept of local vs. remote windows/documents. It's not yet totally clear to me how this should interact with the tree of Page objects. It's possible that Page should become nothing more than a wrapper around a BrowsingContext object.

[jdm]

#10630 is the design I had in mind for this. It's not really worth merging until we actually create instances of the Remote ones.

[KiChjang]

Do we already have this?

[jdm]

No.

[asajeffrey]

We have part of this in #15358.

[nox]

@asajeffrey What's missing?

[jdm]

Actual communication with the cross-origin script threads.

[asajeffrey]

@jdm: we have postMessage, is there other communication you had in mind?

[jdm]

Whoops, I guess not.