script: Move navigation fetching to the ScriptThread (#34919)

This allows reusing the asynchrnous fetch mechanism that we use for page
resources and is likely a step toward removing the `FetchThread`.

Benefits:
 - Reduces IPC traffic during navigation. Now instead of bouncing
   between the constellation and the `ScriptThread` responses are sent
   directly to the `ScriptThread`.
 - Allows cancelling loads after redirects, which was not possible
   before.

There is the question of what to do when a redirect is cross-origin
(#23037). This currently isn't handled properly as the `Constellation`
sends data to the same `Pipeline` that initiated the load. This change
doesn't fix this issue, but does make it more possible for the
`ScriptThread` to shut down the pipeline and ask the `Constellation` to
replace it with a new one.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/fonts/font_context.rs

@@ -661,6 +661,7 @@ impl RemoteWebFontDownloader {
             &core_resource_thread_clone,
             request,
             None,
+            None,
             Box::new(move |response_message| {
                 match downloader.handle_web_font_fetch_message(response_message) {
                     DownloaderResponseResult::InProcess => {},