Allow WebStateObserver to observe N WebStates [42/N].

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

Bug: 775684
Change-Id: Ic880d84c8e0ede5ec1abe06336a47eeee36e05f7
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/778980


Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518284}