Add `base="true"` to pointing stick histograms

isherman@ pointed out that this should be set since these names are only
ever reported with suffixes.

Bug: 1114828
Change-Id: I8d015e50cfbcb23592a28a4f19ec70fdbb02e64f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577836
Auto-Submit: Harry Cutts <hcutts@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834586}

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

@@ -11192,7 +11192,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
-<histogram name="PointingStick.Acceleration" enum="BooleanEnabled"
+<histogram base="true" name="PointingStick.Acceleration" enum="BooleanEnabled"
     expires_after="2021-04-01">
 <!-- Name completed by histogram_suffixes name="PreferenceChangeType" -->
 
@@ -11206,8 +11206,8 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
-<histogram name="PointingStick.PointerSensitivity" enum="PointerSensitivity"
-    expires_after="2021-04-01">
+<histogram base="true" name="PointingStick.PointerSensitivity"
+    enum="PointerSensitivity" expires_after="2021-04-01">
 <!-- Name completed by histogram_suffixes name="PreferenceChangeType" -->
 
   <owner>hcutts@chromium.org</owner>