[sync] Add feature kSyncRememberCustomPassphraseAfterSignout

If a custom passphrase user signs out and back in again, they shouldn't
have to enter their passphrase again. It should get cleared only when
the account is removed from the device, or if Sync is reset from the
dashboard.

To do that, a new gaia-keyed pref will be added in coming CLs.
This CL creates a flag to control these changes. And adds a chrome flag for testing on Bling.

Bug: 1471928
Change-Id: Ie9e96d2e1d0e7236304025e20bfa5f8ff0c0794b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5237035
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Jood Hajeer <jood@google.com>
Cr-Commit-Position: refs/heads/main@{#1252586}