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

This reverts commit 4d11ed9db093fc278985bea63c03f2a7b0a94bd7.

Reason for revert:
LUCI Bisection has identified this change as the cause of a test failure. See the analysis: https://ci.chromium.org/ui/p/chromium/bisection/test-analysis/b/5727830849617920

Sample build with failed test: https://ci.chromium.org/b/8762377901906167601
Affected test(s):
[ninja://:blink_wpt_tests/virtual/no-forced-frame-updates/external/wpt/html/browsers/browsing-the-web/history-traversal/pagereveal/tentative/order-in-prerender-activation.html](https://ci.chromium.org/ui/test/chromium/ninja:%2F%2F:blink_wpt_tests%2Fvirtual%2Fno-forced-frame-updates%2Fexternal%2Fwpt%2Fhtml%2Fbrowsers%2Fbrowsing-the-web%2Fhistory-traversal%2Fpagereveal%2Ftentative%2Forder-in-prerender-activation.html?q=VHash%3Ad66f84fc0745c58a)

If this is a false positive, please report it at http://b.corp.google.com/createIssue?component=1199205&description=Analysis%3A+https%3A%2F%2Fci.chromium.org%2Fui%2Fp%2Fchromium%2Fbisection%2Ftest-analysis%2Fb%2F5727830849617920&format=PLAIN&priority=P3&title=Wrongly+blamed+https%3A%2F%2Fchromium-review.googlesource.com%2Fc%2Fchromium%2Fsrc%2F%2B%2F5069563&type=BUG

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: I422f7671f60bccb99dc0b1dfbc3896a40c2a4210
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5099124
Owners-Override: Daniel d'Andrada <[email protected]>
Reviewed-by: Daniel d'Andrada <[email protected]>
Commit-Queue: Daniel d'Andrada <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1234456}

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))