Bug 1779096 - Clear FontFaceSetImpl::mOwner when destroyed. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D151554

layout/style/FontFaceSetImpl.cpp

@@ -93,10 +93,9 @@ void FontFaceSetImpl::Destroy() {
   }
 
   mLoaders.Clear();
-
   mNonRuleFaces.Clear();
-
   gfxUserFontSet::Destroy();
+  mOwner = nullptr;
 }
 
 void FontFaceSetImpl::ParseFontShorthandForMatching(