UKM: fix typo in Autoplay UKM event metric.

Bug: 744659
Change-Id: I3de1c7b784759ef930239ce40b8391409cf93143
Reviewed-on: https://chromium-review.googlesource.com/746662Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513114}

tools/metrics/ukm/ukm.xml

@@ -583,7 +583,7 @@ be describing additional metrics about the same event.
       Source of the autoplay attempt: 0 for attribute; 1 for play().
     </summary>
   </metric>
-  <metric name="UserGestureRequirement">
+  <metric name="UserGestureRequired">
     <summary>
       Whether a user gesture was required per autoplay rules at the time of
       attempt. By definition there is no user gesture on the stack when the