Revert "Reland "[keepalive-migration] Add fieldtrial testing config""

This reverts commit 8221a2fd6bab0b59029602de6025d8c49947b553.

Reason for revert: Seems to make chrome sporadically omit cookies in request, see crbug.com/1509997

Original change's description:
> Reland "[keepalive-migration] Add fieldtrial testing config"
>
> This is a reland of commit 4d11ed9db093fc278985bea63c03f2a7b0a94bd7
>
> There is another test `virtual/no-forced-frame-updates/external/wpt/html/browsers/browsing-the-web/history-traversal/pagereveal/tentative/order-in-prerender-activation.html` [1]
> that also relies on the pre-migration side effect of
> `PrerenderChannel`.
>
> This patch disable the migration for that virtual test suite.
>
> [1]: https://ci.chromium.org/ui/p/chromium/builders/findit/test-single-revision/4940/test-results?sortby=&groupby=
>
> Original change's description:
> > [keepalive-migration] Add fieldtrial testing config
> >
> > Also explicitly disable the flag for the virtual test suite that
> > relies on the pre-migration side effect of making fetch keepalive
> > requests, that Document being able to stay alive after unload, and
> > hence being able to execute Promise result.
> >
> > Bug: 1356128
> > Change-Id: I330672ab2e40e299d46f48771c665e31755013cc
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5069563
> > Reviewed-by: Xianzhu Wang <[email protected]>
> > Reviewed-by: Tsuyoshi Horo <[email protected]>
> > Auto-Submit: Ming-Ying Chung <[email protected]>
> > Reviewed-by: Hiroki Nakagawa <[email protected]>
> > Commit-Queue: Hiroki Nakagawa <[email protected]>
> > Cr-Commit-Position: refs/heads/main@{#1234386}
>
> Bug: 1356128
> Change-Id: I3ae9a0c23d15dbd0472041816aef50db27b9d112
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5101871
> Reviewed-by: Tsuyoshi Horo <[email protected]>
> Reviewed-by: Kent Tamura <[email protected]>
> Commit-Queue: Ming-Ying Chung <[email protected]>
> Cr-Commit-Position: refs/heads/main@{#1234841}

Bug: 1356128, 1509997
Change-Id: Icbca59c71101253f80319f6cb61070d144930bb3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5104800
Bot-Commit: Rubber Stamper <[email protected]>
Owners-Override: Nicolas Dossou-Gbété <[email protected]>
Reviewed-by: Nicolas Dossou-Gbété <[email protected]>
Reviewed-by: Tsuyoshi Horo <[email protected]>
Commit-Queue: Nicolas Dossou-Gbété <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1235640}

speculation-rules/prerender/resources/utils.js

@@ -29,9 +29,6 @@ class PrerenderChannel extends EventTarget {
       while (this.#active) {
         // Add the "keepalive" option to avoid fetch() results in unhandled
         // rejection with fetch abortion due to window.close().
-        // TODO(crbug.com/1356128): After this migration, "keepalive" will not
-        // be able to extend the lifetime of a Document, such that it cannot be
-        // used here to guarantee the promise resolution.
         const messages = await (await fetch(this.#url, {keepalive: true})).json();
         for (const {data, id} of messages) {
           if (!this.#ids.has(id))