style: Add a mechanism to serialize shorthands for getComputedStyle().

This implements the mechanism reusing the animation machinery for now, so it
asserts in a few cases that this wouldn't handle correctly.

For shorthands that have colors and other bits we'd need a more sophisticated
mechanism with a bit more code (that resolves colors and such), but it'd look
something like this regardless, and we should have this in any case.

Differential Revision: https://phabricator.services.mozilla.com/D11944

components/style/properties/declaration_block.rs

@@ -309,7 +309,9 @@ impl PropertyDeclarationBlock {
             .find(|(declaration, _)| declaration.id() == property)
     }
 
-    fn shorthand_to_css(
+    /// Tries to serialize a given shorthand from the declarations in this
+    /// block.
+    pub fn shorthand_to_css(
         &self,
         shorthand: ShorthandId,
         dest: &mut CssStringWriter,