Add new UndecryptablePasswords study

[bsclifton]

Enables the SkipUndecryptablePasswords feature to help users who are not able to see their logins in brave://password-manager/passwords

Fixes #1164

Main issue tracked with brave/brave-browser#33548

[github-actions]

✅ Test Seed Generated Successfully

To apply the test seed:

1. Desktop: Launch the browser with --variations-pr=1165.
   Android: Set the command line to --variations-pr=1165 in debug menu, restart the browser.
   iOS: Set Variations PR to 1165 in Brave Core Switches debug menu, restart the browser.
2. Wait 5-10 seconds to fetch the seed.
3. Restart the browser to apply the seed.
4. Ensure Active Variations section at brave://version starts with the expected seed version (see below).

Seed Details

Parameter	Value
Version	pull/1165@1004b2a3b3913b382683c993bc6bcfbbc133d090
Uploaded	Thu, 08 Aug 2024 20:57:57 GMT
PR commit	a7e0f9a
Base commit	4f91a05
Merge commit	1004b2a
Serial number	495c23b591d66492e548d6fd168f255a

[github-actions]

The security team is monitoring all repositories for certain keywords. This PR includes the word(s) "password, login" and so security team members have been added as reviewers to take a look.

No need to request a full security review at this stage, the security team will take a look shortly and either clear the label or request more information/changes.

Notifications have already been sent, but if this is blocking your merge feel free to reach out directly to the security team on Slack so that we can expedite this check.

[kjozwiak]

Looks like we pushed out the above without verifying the actual study. Looks like @LaurenWags checked brave/brave-core#25040 via brave/brave-core#25040 (comment) & brave/brave-core#25040 (comment) so we at least didn't merge/push into production blindly.

[bsclifton]

Just for clarification - we've been using the work-around (via CLI; but exact same code) internally since 127 launched.
--enable-features=SkipUndecryptablePasswords

Folks helping on Community have been recommending the same one since then too - we've had a ton of Community threads and Reddit posts.

• https://community.brave.com/t/aggregate-topic-lost-passwords-in-july-2024-update/560154
• https://old.reddit.com/r/brave_browser/comments/1eogk4t/all_passwords_deleted_use_a_password_manager/
• https://old.reddit.com/r/brave_browser/comments/1embulm/help_my_passwords_are_gone_after_update_last_night/ (edited)

Chrome has been live with the exact same feature since Jul 25 (although they did enabled the ClearUndecryptablePasswordsInSync one; which @szilardszaloki and I think is too risky)

• https://issues.chromium.org/issues/354847250#comment33
• https://griffin.brave.com/?seed=UPSTREAM&search=Enable_UndecryptablePasswordsEmergencyFix (edited)