Add VR VoiceSearch UKM

Adding a UKM metric to track whether a page was specifically requested
via voice search.

Bug: 808584
Change-Id: Id20c93ccf3c193e9559a2fbf5bcb87987e08fc1e
Reviewed-on: https://chromium-review.googlesource.com/940605


Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Anna Offenwanger <offenwanger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540272}