LibWeb: Set navigation start time when creating a new Document

This currently uses a non spec-compliant property on the Response
object, which represents the time that the Response was created.

Setting this value allows `Performance.timeOrigin` to return a
reasonable value.

Libraries/LibWeb/Fetch/Infrastructure/HTTP/Responses.h

@@ -125,6 +125,7 @@ public:
 
     // Non-standard
     [[nodiscard]] Optional<StringView> network_error_message() const;
+    MonotonicTime response_time() const { return m_response_time; }
 
 protected:
     explicit Response(GC::Ref<HeaderList>);