I'm adding the UseCounter for handing out a pose when not presenting, but not yet the session timer.

Per discussion, session timer is not necessary anymore, right?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8065da01b71819d56f5a52514c844a9fe9a65685

commit 8065da01b71819d56f5a52514c844a9fe9a65685
Author: Bill Orr <billorr@chromium.org>
Date: Thu Mar 15 23:20:33 2018

Add UseCounter for getFrameData.

This will allow us to determine how much magic window mode is used vs
presentation by looking at the ratio of this UseCounter vs presentation
UseCounter.

TBR=vollick@ for web_feature.mojom, which doesn't need owner review.
BUG=776086

Change-Id: I6e28433a00a8dbb9be3315d926d8f2b225be5b3e
Reviewed-on: https://chromium-review.googlesource.com/752079
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543541}
[modify] https://crrev.com/8065da01b71819d56f5a52514c844a9fe9a65685/third_party/WebKit/Source/modules/vr/VRDisplay.idl
[modify] https://crrev.com/8065da01b71819d56f5a52514c844a9fe9a65685/third_party/WebKit/public/platform/web_feature.mojom
[modify] https://crrev.com/8065da01b71819d56f5a52514c844a9fe9a65685/tools/metrics/histograms/enums.xml

@offenwanger : is this something that'll be wrapped up soon? I don't think we're going to get it in for M67 nor would I hold M67 for this. moving to 68...but if no additional work will be done on this soon maybe we should clear milestone.

Removing Blink>WebVR component and assigning to Blink>WebXR 

Removing Blink>WebVR component and assigning to Blink>WebXR 

With the above change we are collecting some passive data on magic window useage, but it would be good to get some more explicit data. 
Also, the above change only covers the WebVR api, not the WebXR api.