Add watch time metrics for HTML5 media playback.

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 4e7cabfaed943d03d6a74f51f929b279e8788ce3..afe10ba798ca94457eed1c658dedb72767fea0f4 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -23152,6 +23152,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.WatchTime">
+  <owner>dalecurtis@chromium.org</owner>
+  <summary>
+    Elapsed watch time for a single HTML5 audio/video playback session.

[sandersd (OOO until July 31), 2016/08/03 22:45:59]

Put some of the explanation from the CL description here?

[DaleCurtis, 2016/08/04 19:14:59]

Done, I elided some of the time constants though.

+  </summary>
+</histogram>
+
 <histogram name="Media.WindowsCoreAudioInput" enum="BooleanSuccess">
   <owner>henrika@chromium.org</owner>
   <summary>
@@ -97706,6 +97713,24 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="Media.VideoCaptureManager"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="MediaWatchTimeCategories" separator=".">
+  <suffix name="AudioVideo.All"
+      label="Watch time for all media with both an audio and video track."/>
+  <suffix name="AudioVideo.AC"
+      label="Watch time for all media with both an audio and video track on
+             AC power."/>
+  <suffix name="AudioVideo.Battery"
+      label="Watch time for all media with both an audio and video track on
+             battery power."/>
+  <suffix name="AudioVideo.MSE"
+      label="Watch time for MSE media with both an audio and video track."/>
+  <suffix name="AudioVideo.SRC"
+      label="Watch time for SRC media with both an audio and video track."/>
+  <suffix name="AudioVideo.EME"
+      label="Watch time for EME media with both an audio and video track."/>
+  <affected-histogram name="Media.WatchTime"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="MobileDownloadBytesDownloadedTypes" separator=".">
   <suffix name="ChromeNetworkStack.Failure"/>
   <suffix name="ChromeNetworkStack.Success"/>