Update WebXR ReferenceSpace.Requested Histogram

The XR.WebXR.ReferenceSpace.Requested histogram was set to expire in
December. This change updates it's expiry to that of it's somewhat
paired histogram, XR.WebXR.ReferenceSpace.Succeeded.

Fixed: 1144802
Change-Id: I49f4847f0893eaa0441e805ef3d626c0e7e6bc5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2514183
Auto-Submit: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823336}

tools/metrics/histograms/histograms_xml/others/histograms.xml

@@ -17062,7 +17062,7 @@ regressions. -->
 </histogram>
 
 <histogram name="XR.WebXR.ReferenceSpace.Requested" enum="XRReferenceSpaceType"
-    expires_after="2020-12-13">
+    expires_after="2021-03-15">
   <owner>alcooper@chromium.org</owner>
   <owner>xr-dev@chromium.org</owner>
   <summary>