Improve description of Media.AudioInputControllerSessionSilenceReport metric.

Bug: None
Change-Id: Iaa2da4e58fe8605cbccbd8d204eafe0689de2387
Reviewed-on: https://chromium-review.googlesource.com/721039Reviewed-by: Olga Sharonova <olka@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Henrik Grunell <grunell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509700}

tools/metrics/histograms/histograms.xml

@@ -31674,7 +31674,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="Media.AudioInputControllerSessionSilenceReport"
     enum="AudioInputSilenceReport">
   <owner>grunell@chromium.org</owner>
-  <summary>The silence report for an audio input device session.</summary>
+  <summary>
+    Every 15 seconds during the lifetime of an audio input stream, the sound
+    level is measured, see AudioInputController::CheckAudioPower(). If the level
+    is below a certain threshold, it's considered &quot;silence&quot;. Otherwise
+    it's considered &quot;audio&quot;. This metric shows whether there was only
+    silence, only audio, or both, during the lifetime of a stream. If logged
+    before the first measurement, i.e. if the lifetime is less than 15 seconds,
+    &quot;no measurement&quot; is reported. If there is no audio data provided
+    by the OS during the lifetime of the stream, for example because of a bug,
+    measurements will not be performed and &quot;no measurement&quot; is
+    reported.
+  </summary>
 </histogram>
 
 <histogram name="Media.AudioInputDeviceManager" units="ms">