Auto merge of #15710 - canaltinova:clip, r=emilio

Implement gecko glue for clip property

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Implemented gecko glue for clip property.

Clip property looks slightly different in gecko. `auto` top and left values are preserved as auto in [gecko](http://searchfox.org/mozilla-central/rev/39e4b25a076c59d2e7820297d62319f167871449/layout/style/nsRuleNode.cpp#10294,10316) but converted to 0 in [servo](https://dxr.mozilla.org/servo/rev/65624dbfc28442b58145215f524eb13aeb2cadf6/components/style/values/specified/mod.rs#942). Gecko is setting `NS_STYLE_CLIP_TOP_AUTO` and `NS_STYLE_CLIP_LEFT_AUTO` flags with that information. But I tried this property in stylo build and it is working correctly like this. It looks like it doesn't change the outcome of the property.
~Do we really need to set these flags?~
Manishearth and bz said that auto and 0 values are same.

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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 1341728](https://bugzilla.mozilla.org/show_bug.cgi?id=1341728)

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- [X] These changes do not require tests because this is stylo side change.

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