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The updates to the back/forward buttons, related to the tab history
presentations are now moved to a new object. The goal of this object is
to contain most of the updates of the buttons of the toolbar.

Bug: 784846
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Idc95d91cfca5a265e661d89550a8b1dc5e40d3ef
Reviewed-on: https://chromium-review.googlesource.com/768869
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517440}