Migrate components/password_manager to SyncUserSettings

SyncUserSettings is a new class that encapsulates all the
user-configurable knobs for Sync. It replaces a bunch of setters
and getters directly on the SyncService.

While I'm here, also
- update a comment in GetSyncUsernameIfSyncingPasswords to be more
  specific, since the semantics here are a bit weird, and
- remove a CanSyncFeatureStart check from
  ShouldAffiliationBasedMatchingBeActive since it's redundant with the
  check for IsSyncFeatureActive which is strictly stronger.

Bug: 884159
Change-Id: Ibbb9a82a2d89520e1a349e29a1050cd84dd52ea5
Reviewed-on: https://chromium-review.googlesource.com/c/1344096
Reviewed-by: Vaclav Brozek <vabr@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609964}
5 files changed