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The custom bottom toolbar is removed in favor of the toolbar already built into
UINavigationController. Showcase support is removed because this container is no
longer materially different from a stock UINavigationController.

BUG=805013,805201

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ib8e78793ea73c411ad8c72cabe18568c4cb71840
Reviewed-on: https://chromium-review.googlesource.com/1015541Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551709}