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Added a `TopLevelBrowsingContextId` type, which is a subtype of `BrowsingContextId` that only refers to top-level browsing contexts.
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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 do not require tests because refactoring
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Source-Repo: https://github.com/servo/servo
Source-Revision: a0d6d6883c3f2fde6f0f3e13712a569ce3aa41e5
cc87e16b97