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>
