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r? @asajeffrey
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<!-- 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 fix#5670 (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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implement go, forward, back
Source-Repo: https://github.com/servo/servo
Source-Revision: 0e887ca8d3327b63db43cc329dd83bf4c02e2a1c
