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This CL adds a new ChromeBroadcastObserver method for the toolbar
height.  It also creates FullscreenUIState, an object that holds the
broadcast properties for toolbar UI.

Bug: 785625
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ibebff64db6c33ec5e7e0ca6eb24a96973a533b36
Reviewed-on: https://chromium-review.googlesource.com/773443
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519526}