Preserve sourceURL comment on style sheets

In addition to the sourceMappingURL comment, there is a second special
comment, "sourceURL", that can be used to set the "display name" of a
style sheet for developer tools.  This name is also used as the base
URL for the source-map URL resolution algorithm.  sourceURL is
described here:
https://blog.getfirebug.com/2009/08/11/give-your-eval-a-name-with-sourceurl/
The devtools feature bug is here:
https://bugzilla.mozilla.org/show_bug.cgi?id=880831

This patch changes servo to preserve and expose this value for use in M-C.

components/script/stylesheet_loader.rs

@@ -292,6 +292,7 @@ impl<'a> StyleStylesheetLoader for StylesheetLoader<'a> {
                 quirks_mode: context.quirks_mode,
                 namespaces: RwLock::new(Namespaces::default()),
                 source_map_url: RwLock::new(None),
+                source_url: RwLock::new(None),
             },
             media: media,
             shared_lock: lock.clone(),