Auto merge of #18212 - mrobinson:position-sticky, r=emilio

Add support for position:sticky

This leverages the position:sticky support in WebRender to bring basic
support for position:sticky in Servo. There are still some issues with
nested sticky flows as well as a few other corner cases. Tests are
imported from WPT and can be removed once we update to the latest
version.

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components/style/properties/longhand/box.mako.rs

@@ -203,8 +203,7 @@ ${helpers.single_keyword("-moz-top-layer", "none top",
                          products="gecko", animation_value_type="none", internal=True,
                          spec="Internal (not web-exposed)")}
 
-${helpers.single_keyword("position", "static absolute relative fixed",
-                         extra_gecko_values="sticky",
+${helpers.single_keyword("position", "static absolute relative fixed sticky",
                          animation_value_type="discrete",
                          flags="CREATES_STACKING_CONTEXT ABSPOS_CB",
                          spec="https://drafts.csswg.org/css-position/#position-property")}