style: Alias -webkit-clip-path to clip-path for webcompat

Differential Revision: https://phabricator.services.mozilla.com/D162674

components/style/properties/counted_unknown_properties.py

@@ -19,7 +19,6 @@ COUNTED_UNKNOWN_PROPERTIES = [
     "text-size-adjust",
     "-webkit-font-feature-settings",
     "-webkit-user-drag",
-    "-webkit-clip-path",
     "orphans",
     "widows",
     "-webkit-user-modify",

components/style/properties/longhands/svg.mako.rs

@@ -80,6 +80,7 @@ ${helpers.predefined_type(
     "basic_shape::ClipPath",
     "generics::basic_shape::ClipPath::None",
     engines="gecko",
+    extra_prefixes="webkit",
     animation_value_type="basic_shape::ClipPath",
     spec="https://drafts.fxtf.org/css-masking/#propdef-clip-path",
 )}