[Spec] Spec embedder initiated fenced frame navigation flag in navigation request

Spec the added member `is_embedder_initiated_fenced_frame_navigation_` in NavigationRequest. See chromium [implementation](https://source.chromium.org/chromium/chromium/src/+/main:content/browser/renderer_host/navigation_request.h?q=symbol%3A%5Cbcontent%3A%3ANavigationRequest%3A%3Ais_embedder_initiated_fenced_frame_navigation_%5Cb%20case%3Ayes).