We populate this field from `network::ResourceRequest`s
`top_frame_origin`, but it is being phased out in favor
of `net::NetworkIsolationKey`. The transition has not fully
completed yet, so in some cases `top_frame_origin` is populated
while in others `trusted_network_isolation_key` is.

cc: @iefremov

Chromium change:

https://chromium.googlesource.com/chromium/src/+/1b5366c62c02fd6edfe83703e57e4fcd14d38f64

commit 1b5366c62c02fd6edfe83703e57e4fcd14d38f64
Author: Shivani Sharma <shivanisha@chromium.org>
Date:   Tue Jul 16 20:08:13 2019 +0000

    Split cache: Network Isolation key for navigation resources

    Adds network isolation key in the ResourceRequest for navigations to be
    propagated to net::URLRequest. The key is also updated for redirects.

    Updates NavigationRequestInfo to have a network isolation key as a
    member instead of top frame origin. Also updated the corresponding
    intercepting browser tests in navigation_browsertest.

    Added browser tests to verify if the navigation resources (both main
    frame and sub frame) are expected to be cached including the redirect use
    case.

    Also removing the code in url_request_http_job.cc that was using top frame
    origin to create NIK if it was not already populated. This is because with
    this change we would have NIK being created in browser process for subresources,
    navigation resources and dedicated worker resources which were earlier using the
    top frame origin.

    (Note that sub-resources URL requests do not use this path and have their
    network isolation key filled up in the URLLoaderFactory).

    Bug: 984130