style: Use the owned slice type for basic shape polygon coordinates.

This enables destructors for tagged unions in cbindgen, implemented in:

 * https://github.com/eqrion/cbindgen/pull/333

Which allow us to properly generate a destructor for the cbindgen-generated
StyleBasicShape (which now contains an OwnedSlice).

For now, we still use the glue code to go from Box<BasicShape> to
UniquePtr<BasicShape>. But that will change in the future when we generate even
more stuff and remove all the glue.

I could add support for copy-constructor generation to cbindgen for tagged
enums, but I'm not sure if it'll end up being needed, and copy-constructing
unions in C++ is always very tricky.

Differential Revision: https://phabricator.services.mozilla.com/D29769

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

@@ -87,8 +87,8 @@ ${helpers.predefined_type(
     "basic_shape::ClippingShape",
     "generics::basic_shape::ShapeSource::None",
     products="gecko",
-    boxed=True,
     animation_value_type="basic_shape::ClippingShape",
+    boxed=True,
     flags="CREATES_STACKING_CONTEXT",
     spec="https://drafts.fxtf.org/css-masking/#propdef-clip-path",
 )}