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With this change, the voice search button is disabled via binding when
we are in incognito mode. Additionally, we store a bit on the model for
when we've got experimental VR features enabled so that it may more
easily be bound and toggled in tests. This also means that the ui scene
manager no longer needs to retain a pointer to the voice search button.
Further, we no longer add the button to control_elements_ since it will
inherit the visibility of the URL bar due to being in its subtree.

Bug: 779592
Change-Id: I92832f46b4cdf33e7a9caf57076001a24ff9ab83
Reviewed-on: https://chromium-review.googlesource.com/748774
Commit-Queue: Ian Vollick <vollick@chromium.org>
Reviewed-by: Biao She <bshe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513233}