Allow WebStateObserver to observe N WebStates [35/N].

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

Bug: 775684
Change-Id: Ifdf8f2ba5e113f67269d4329500d1693e772eeae
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/768877
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#517793}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ef11aff0688d84898e71c7d6021f0e51109d416e
1 file changed