Interpolated result of FontWeight is wrong because clamping code is incorrect.
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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 [Bug 1387948](https://bugzilla.mozilla.org/show_bug.cgi?id=1387948).
- [X] These changes do not require tests because Gecko has tests
Source-Repo: https://github.com/servo/servo
Source-Revision: 48b7e6d27c6fbeea9b03acface18d163cb7eedbc