Add audio only watch time metrics.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 4e1dd217e61e72b9e2e5c2caa24e46af81b4cdd4..71b35323cdf017770e130c69c6d1cc54edf2bff8 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -108077,6 +108077,19 @@ value.
 </histogram_suffixes>
 
 <histogram_suffixes name="MediaWatchTimeCategories" separator=".">
+  <suffix name="Audio.All"
+      label="Watch time for all media with only an audio track."/>
+  <suffix name="Audio.AC"
+      label="Watch time for all media with only an audio track on AC power."/>
+  <suffix name="Audio.Battery"
+      label="Watch time for all media with only an audio track on battery
+             power."/>
+  <suffix name="Audio.MSE"
+      label="Watch time for MSE media with only an audio track."/>
+  <suffix name="Audio.SRC"
+      label="Watch time for SRC media with only an audio track."/>
+  <suffix name="Audio.EME"
+      label="Watch time for EME media with only an audio track."/>
   <suffix name="AudioVideo.All"
       label="Watch time for all media with both an audio and video track."/>
   <suffix name="AudioVideo.AC"