Allow WebStateObserver to observe N WebStates [43/N].

Convert PasswordController to directly track the registration
with the observed WebState instead of relying on the deprecated
code in WebStateObserverBridge.

Bug: 775684
Change-Id: I227193a0a4040cff7a823c4e5926a3cad78650e6
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Reviewed-on: https://chromium-review.googlesource.com/778900


Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518308}