Auto merge of #13663 - shubheksha:fix/13657, r=Manishearth

changed set_value() to set() in gecko.mako.rs

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Changed `set_value()` to `set()` in gecko.mako.rs

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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 #13657 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because just the `set_value()` call was changed to `set()`

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