[UPM] Adding a feature flag for kClearLoginDatabaseForUPMUsers

This change allows to control kClearLoginDatabaseForUPMUsers feature
through chrome://flags which simplifies testing.

Fixed: b/328029669
Change-Id: I10c17441362183fae64a0a36dbc6a91aa5f62643
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5427895


Reviewed-by: Ivana Žužić <izuzic@google.com>
Commit-Queue: Viktor Semeniuk <vsemeniuk@google.com>
Cr-Commit-Position: refs/heads/main@{#1283107}