Bug 1423832: Make sure to handle gracefully rule notifications when the styleset has been detached already. r=heycam

MozReview-Commit-ID: xA8dYJX2P6

--HG--
extra : rebase_source : 18b4c491785367c9bd04604fcaf2d22f5fea1762

layout/style/ServoStyleSet.cpp

@@ -1036,6 +1036,10 @@ ServoStyleSet::StyleNewSubtree(Element* aRoot)
 void
 ServoStyleSet::MarkOriginsDirty(OriginFlags aChangedOrigins)
 {
+  if (MOZ_UNLIKELY(!mRawSet)) {
+    return;
+  }
+
   SetStylistStyleSheetsDirty();
   Servo_StyleSet_NoteStyleSheetsChanged(mRawSet.get(),
                                         mAuthorStyleDisabled,