Auto merge of servo#17922 - upsuper:wk-dpr, r=heycam

stylo: Support layout.css.prefixes.device-pixel-ratio-webkit pref

This is the Servo side change of [bug 1366956](https://bugzilla.mozilla.org/show_bug.cgi?id=1366956).

Auto merge of servo#16962 - hiikezoe:prefixed-intrinsic-size-value, r…

…=Manishearth

Prefixed intrinsic size value

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This is a PR for https://bugzilla.mozilla.org/show_bug.cgi?id=1355402
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix servo#16788
- [X] These changes do not require tests because it's for stylo

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