Revert "[PasswordManager] Stop Using Native Backends on Linux systems"

This reverts commit 6811de35a39128fb354f238bce350f9a4a643e9d.

Reason for revert: https://bugs.chromium.org/p/chromium/issues/detail?id=988010

Original change's description:
> [PasswordManager] Stop Using Native Backends on Linux systems
> 
> Recent migrations moved all users who have been using the keying to the
> LoginDatabase. This CL forces all users to using LoginDatabase
> instead of native backends on Linux.
> 
> TBR=pcc@chromium.org
> 
> Change-Id: Ibac11e68d8f354ecf7f3c9828d95777ea5b3df60
> Bug: 950269, 950267
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1713560
> Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
> Reviewed-by: Peter Collingbourne <pcc@chromium.org>
> Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
> Reviewed-by: Christos Froussios <cfroussios@chromium.org>
> Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#681042}

TBR=vasilii@chromium.org,pcc@chromium.org,cfroussios@chromium.org,mamir@chromium.org

Change-Id: I2a42c76df8380cdaafb3485576e1efb9735f9d8d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 950269, 950267
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1721073
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681328}