CRAS: add histograms for number of underruns

The change is related to CL:1148178. Add histogram infomation in
histograms.xml to make the histogram visible on chromedashboard.

BUG=b:77485214

Change-Id: I4cc97b06038b5736eb42bf169a8e91e2674ea4ad
Reviewed-on: https://chromium-review.googlesource.com/1154742
Commit-Queue: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580047}

tools/metrics/histograms/histograms.xml

@@ -14760,6 +14760,15 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="Cras.UnderrunsPerDevice" units="count">
+  <owner>yuhsuan@chromium.org</owner>
+  <summary>
+    The number of underruns happened on one audio device. Underrun here means
+    there are no samples in device buffer while device is playing. It is
+    recorded when device is closed by CRAS(Chrome OS audio server).
+  </summary>
+</histogram>
+
 <histogram name="CrashExitCodes.Extension" enum="CrashExitCodes">
   <owner>wfh@chromium.org</owner>
   <summary>