Bug 1719963 - Restore EnsureUniqueInner() call for now. r=jfkthame

This effectively reverts the behavior to the one before bug 1711437
(making the CC setup sound again), but without a big backout.

Fixing the CC setup of @import rules properly is a bit more involved
than what I anticipated and I don't want to have DevTools folks blocked
for too long, nor have this crash in-tree for too long either.

Differential Revision: https://phabricator.services.mozilla.com/D122819

layout/style/StyleSheet.cpp

@@ -1385,6 +1385,10 @@ already_AddRefed<StyleSheet> StyleSheet::CloneAdoptedSheet(
 
 ServoCSSRuleList* StyleSheet::GetCssRulesInternal() {
   if (!mRuleList) {
+    // TODO(emilio): This should go away, but we need to fix the CC setup for
+    // @import rules first, see bug 1719963.
+    EnsureUniqueInner();
+
     RefPtr<ServoCssRules> rawRules =
         Servo_StyleSheet_GetRules(Inner().mContents).Consume();
     MOZ_ASSERT(rawRules);