Reland "Add IPH_SignInBenefits to UnoPhase2Desktop fieldtrial config"

This reverts commit 7d8a6ecdd2685f9035aa4ded2586c0dd85ea17ad.

Reason for revert: Replaced the failing CHECK with a conditional return.

Original change's description:
> Revert "Add IPH_SignInBenefits to UnoPhase2Desktop fieldtrial config"
>
> This reverts commit e7ed3f593ac47f2c1bdbc93ca7c7b14ac3ec1559.
>
> 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/5486798831616000
>
> Sample build with failed test: https://ci.chromium.org/b/8702201538782093265
> Affected test(s):
> [ninja://:chrome_wpt_tests/external/wpt/webdriver/tests/bidi/network/add_data_collector/user_contexts.py](https://ci.chromium.org/ui/test/chromium/ninja:%2F%2F:chrome_wpt_tests%2Fexternal%2Fwpt%2Fwebdriver%2Ftests%2Fbidi%2Fnetwork%2Fadd_data_collector%2Fuser_contexts.py?q=VHash%3A1b1e9c5537be9730)
>
> 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%2F5486798831616000&format=PLAIN&priority=P3&title=Wrongly+blamed+https%3A%2F%2Fchromium-review.googlesource.com%2Fc%2Fchromium%2Fsrc%2F%2B%2F6999653&type=BUG
>
> Original change's description:
> > Add IPH_SignInBenefits to UnoPhase2Desktop fieldtrial config
> >
> > Bug: 448577737
> > Change-Id: If9e8165e193c79a799731b4f2af48b4659fd9657
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6999653
> > Commit-Queue: Ankush Singh <ankushkush@google.com>
> > Reviewed-by: Duong Dac <ddac@chromium.org>
> > Cr-Commit-Position: refs/heads/main@{#1523470}
> >
>
> Bug: 448577737
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Change-Id: I3bee3487e2301ef162d71f93fa85a2810b20d87d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7004027
> Reviewed-by: Nick Birnie <birnie@google.com>
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Commit-Queue: Nick Birnie <birnie@google.com>
> Cr-Commit-Position: refs/heads/main@{#1523811}

Bug: 448577737
Change-Id: I5977d06512911def91c81de1c88cc32d80732dcd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7006086
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Duong Dac <ddac@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1524155}