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/constellation/tracing.rs

@@ -150,7 +150,6 @@ mod from_script {
                 },
                 Self::ScheduleBroadcast(..) => target!("ScheduleBroadcast"),
                 Self::ForwardToEmbedder(msg) => msg.log_target(),
-                Self::InitiateNavigateRequest(..) => target!("InitiateNavigateRequest"),
                 Self::BroadcastStorageEvent(..) => target!("BroadcastStorageEvent"),
                 Self::ChangeRunningAnimationsState(..) => target!("ChangeRunningAnimationsState"),
                 Self::CreateCanvasPaintThread(..) => target!("CreateCanvasPaintThread"),