Auto merge of #14312 - asajeffrey:script-discard-documents, r=cbrewster

Implement discarding Document objects to reclaim space.

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This PR implements document discarding. Active documents are kept alive strongly, but inactive documents are only kept alive weakly. When a document is GCd, it is marked as discarded, and if it is every reactivated, a reload of the URL is triggered.

Note that this PR is pretty aggressive about discarding, and can any inactive document (other than those being kept alive by other same-origin pipelines). We might want to damp it down a bit.

Also note that this interacts with browser.html in that the reloading triggered by reactivating a document triggers mozbrowser events.

To test this, I added a `-Zdiscard-inactive-documents` debug flag, which discards all inactive documents, even ones which are reachable through other same-origin pipelines.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14262.
- [X] These changes do not require tests because we should be able to use the existing tests with `-Zdiscard-inactive-documents`.

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