style: Cache substituted values from shorthand properties during the cascade.

This brings the time down to 1.6ms from 4.8ms on the test-case in the
bug. This should be improvable too, but I think this is a nice
improvement for regular styling as well.

Differential Revision: https://phabricator.services.mozilla.com/D105187

components/style/properties/declaration_block.rs

@@ -837,6 +837,7 @@ impl PropertyDeclarationBlock {
                         custom_properties.as_ref(),
                         QuirksMode::NoQuirks,
                         device,
+                        &mut Default::default()
                     )
                     .to_css(dest)
             },