[Password Manager] Extend metrics for the deprecation of linux backends

The long tail of clients is still migrating. We are keeping the migration
on and will continue to monitor adoption.

Change-Id: I0d48a87d3022fd1828b159078420cec062f261c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1598810
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658478}