Bug 1565922. When the document viewer loses it prescontext pointer we need to make sure that any potential subdocument display items get rebuilt. r=mattwoodrow

If the subdocument display item gets created when the subdocument doesn't have a root frame it'll be based on the subdocument frame in the parent document. This means that the prescontext pointer on the document viewer of the subdocument can go away without the subdocument display item getting any kind of invalidation. So we make sure the subdocument display item gets rebuilt if that happens. The reason the prescontext pointer is important is that is what nsSubDocumentFrame::GetSubdocumentPresShellForPainting uses (via nsDocShell::GetPresShell) to get the subdocument presshell. So it will fail to get a presshell and RetainedDisplayListBuilder::IncrementSubDocPresShellPaintCount will crash.

Differential Revision: https://phabricator.services.mozilla.com/D39310

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extra : moz-landing-system : lando