We should at least enable the VRViewerType metrics for Windows devices in M64.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2c5b9f7876f0b96bc0af38b197a81d6a93b22f5f

commit 2c5b9f7876f0b96bc0af38b197a81d6a93b22f5f
Author: Bill Orr <billorr@chromium.org>
Date: Thu Nov 30 23:43:20 2017

Make VRViewerType metric cross-platform

This change is a targetted fix to send the VRViewerType metric from
multiple locations - both keeping the existing Android-only location
in VrShellGl, and adding a new location in OpenVRDevice.

The two metrics will trigger when we enter VR through the respective
platform-specific VR entry points. These two entrypoints will be
reconciled in a subsequent change that moves more VR metrics to a
cross-platform location.

BUG= 782865 

Change-Id: I17505eaa9bdefccb275dba0f5863a248c78c5968
Reviewed-on: https://chromium-review.googlesource.com/798028
Commit-Queue: Bill Orr <billorr@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520751}
[modify] https://crrev.com/2c5b9f7876f0b96bc0af38b197a81d6a93b22f5f/chrome/browser/android/vr_shell/vr_metrics_util.cc
[modify] https://crrev.com/2c5b9f7876f0b96bc0af38b197a81d6a93b22f5f/chrome/browser/android/vr_shell/vr_metrics_util.h
[modify] https://crrev.com/2c5b9f7876f0b96bc0af38b197a81d6a93b22f5f/device/vr/openvr/openvr_device.cc
[modify] https://crrev.com/2c5b9f7876f0b96bc0af38b197a81d6a93b22f5f/device/vr/vr_device.h
[modify] https://crrev.com/2c5b9f7876f0b96bc0af38b197a81d6a93b22f5f/tools/metrics/histograms/enums.xml