Auto merge of #17875 - bzbarsky:first-line-dual-inheritance, r=emilio

Add support for having two separate parent styles.  Fixes gecko bug 1382806.

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This is needed for ::first-line support.  See https://drafts.csswg.org/css-pseudo-4/#first-line-inheritance

This PR doesn't quite implement what the CSS spec draft says right now.  It implements what Gecko does, which is what an earlier draft said.

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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 https://bugzilla.mozilla.org/show_bug.cgi?id=1382806

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because servo doesn't support ::first-line yet

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