Bug 1417709, round two
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These are the servo changes that need to go with [bug 1417709](https://bugzilla.mozilla.org/show_bug.cgi?id=1417709).
r? @emilio
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).
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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because without these changes Firefox doesn't build.
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