style: Assert we never style a root element from another document.

Right now when calling getComputedStyle with an element from another document,
we return the style using the pres context of that document, not of the document
of the window getComputedStyle was called on, so this holds.

But it will stop holding if we ever change this, so assert it doesn't happen.

Bug: 1374062
Reviewed-By: Manishearth
MozReview-Commit-ID: 3g8yQWWdsen

components/style/matching.rs

@@ -274,6 +274,7 @@ trait PrivateMatchMethods: TElement {
         if self.is_native_anonymous() || cascade_target == CascadeTarget::EagerPseudo {
             cascade_flags.insert(PROHIBIT_DISPLAY_CONTENTS);
         } else if self.is_root() {
+            debug_assert!(self.owner_doc_matches_for_testing(shared_context.stylist.device()));
             cascade_flags.insert(IS_ROOT_ELEMENT);
         }