LibWeb: Add missing check in CrossOriginGetOwnPropertyHelper

We have to check that the entry in CrossOriginProperties is the one
actually requested from the caller before executing the body of the
loop. This fixes a crash triggered by YouTube iframe embedding.

Userland/Libraries/LibWeb/HTML/CrossOrigin/AbstractOperations.cpp

@@ -102,8 +102,17 @@ Optional<JS::PropertyDescriptor> cross_origin_get_own_property_helper(Variant<HT
         .property_key = property_key,
     };
 
+    // SameValue(e.[[Property]], P) can never be true at step 2.1 if P is not a string due to the different type, so we can return early.
+    if (!property_key.is_string()) {
+        return {};
+    }
+    auto const& property_key_string = property_key.as_string();
+
     // 2. For each e of CrossOriginProperties(O):
     for (auto const& entry : cross_origin_properties(object_const_variant)) {
+        if (entry.property != property_key_string)
+            continue;
+        // 1. If SameValue(e.[[Property]], P) is true, then:
         auto& cross_origin_property_descriptor_map = object.visit([](auto* o) -> CrossOriginPropertyDescriptorMap& { return o->cross_origin_property_descriptor_map(); });
 
         // 1. If the value of the [[CrossOriginPropertyDescriptorMap]] internal slot of O contains an entry whose key is crossOriginKey, then return that entry's value.