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This PR will support OpenEndedDictionary (based on MozMap implementation from #13332) in the Headers API.
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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#11897 (github issue number if applicable).
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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____
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Source-Repo: https://github.com/servo/servo
Source-Revision: 06f7a642cf3ebda5ca43b048e2458940b5202c64