Tracks GVR version crossed with headset type using UMA.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 7b2303dccb9969dc8a781e8db719e9c50284764e..b9fcaa08a192267f02962d27227afa9550b8cb91 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -78791,6 +78791,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="VRRuntimeVersion.GVR" units="normalized version">

[ddorwin, 2017/05/25 05:14:12]

When we support other platforms, we would need the histogram name to be
VRRuntimeVersion with suffixes of, for example, OpenVR. Thus, should the
suffixes be "GVR.Cardboard"? I don't know whether that's possible, but I think
that's what we want. And if it's not, how would we handle things not under
".GVR"?

+  <owner>tiborg@chromium.org</owner>
+  <summary>
+    The GVR version being used for VR. Special values: (-4) Could not encode GVR
+    version; (-3) Device does not support VR; (-2) GVR is not installed; (-1)
+    GVR is installed but version is too old to be logged.
+  </summary>
+</histogram>
+
 <histogram name="VRSessionNavigationCount">
   <owner>billorr@chromium.org</owner>
   <summary>
@@ -92116,6 +92125,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <affected-histogram name="WebRTC.Video.Encoded.Qp"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="VRRuntimeVersionGVRHeadset" separator=".">
+  <suffix name="Cardboard" label="GVR version used with Carboard."/>
+  <suffix name="Daydream" label="GVR version used with Daydream."/>
+  <suffix name="Unknown" label="GVR version used with an unknown headset."/>
+  <affected-histogram name="VRRuntimeVersion.GVR"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="VRSessionType" separator=".">
   <suffix name="Browser"
       label="The session is restricted to the period that the browser is