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This is in preparation for a FakeWebViewTranslateClient so unit tests
can be written for the rest of the public methods.

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I7bb15cf38467b5a2b388fee7849599c1876ffa6c
Reviewed-on: https://chromium-review.googlesource.com/953824Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: John Wu <jzw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541669}