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This change defines Graphics, Input, and Scheduler delegates that can
interact with BrowerRenderer to render in-headset VR UI.

With this change, the delegates are mostly defined and parts necessary
to render are implemented, but VR UI doesn't yet actually render any UI
on desktop.

BUG=819448

Change-Id: I8d50e200e2380ae2113454eaf77cd1c53f369554
Reviewed-on: https://chromium-review.googlesource.com/c/1306402
Commit-Queue: Bill Orr <billorr@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605897}