Remove Media.UnderflowCount, add Media.UnderflowDuration for MSE.

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 86869a435ce1fb5ee9a688b389ca56f5a3779b27..e94c81ce2a7307b92a9cfa85a89d150028123adc 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -26473,6 +26473,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="Media.UnderflowCount">
+  <obsolete>
+    Removed Feb 2017. Media.UnderflowDuration provides more useful counts.
+  </obsolete>
   <owner>dalecurtis@chromium.org</owner>
   <summary>
     The number of times a src= playback has underflowed; i.e. ran out of data.
@@ -26482,7 +26485,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="Media.UnderflowDuration" units="ms">
   <owner>dalecurtis@chromium.org</owner>
   <summary>
-    The amount of time taken to leave the underflow state; i.e. resume playback.
+    The amount of time taken to leave the underflow state (i.e. resume playback)
+    for src= playbacks.
+  </summary>
+</histogram>
+
+<histogram name="Media.UnderflowDuration.MSE" units="ms">
+  <owner>dalecurtis@chromium.org</owner>
+  <summary>
+    The amount of time taken to leave the underflow state (i.e. resume playback)
+    for Media Source Extensions (MSE) based playbacks.
   </summary>
 </histogram>