servo: Merge #14013 - Script thread with no root document (from asajeffrey:script-thread-no-root-document); r=jdm
<!-- Please describe your changes on the following line: --> This PR removes the single root document from the script thread, and replaces it by a lookup table from `PipelineId`s to `Document`s. This is needed if we're going to share script threads, as per #633. The last commit is the one that matters, the ones before are #13646. cc @jdm @Ms2ger @ConnorGBrewster --- <!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: --> - [X] `./mach build -d` does not report any errors - [X] `./mach test-tidy` does not report any errors - [X] These changes do not require tests because refactoring <!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. --> Source-Repo: https://github.com/servo/servo Source-Revision: d1b7f19410cb882280287b482bd7178a6eb529b1
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