Add metrics regarding concurrent audible tabs in Chromium.

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 d0803377f04c166d937882c86ea14e50b4b0f4b1..a0c788b861272498e105936982a57669c89ba987 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -18201,6 +18201,32 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.Audible.ConcurrentTabsInSession">
+  <owner>mlamouri@chromium.org</owner>
+  <summary>
+    Records how many tabs have been audible at the same time during the session.
+    It is recording the maximum audible tab everytime it increases. In other

[whywhat, 2016/01/15 19:01:20]

nit: s/tab/tab count/

[mlamouri (slow - plz ping), 2016/01/19 12:18:43]

Done.

+    words, a session with N concurrent audible tabs will record for [1;N].
+  </summary>
+</histogram>
+
+<histogram name="Media.Audible.ConcurrentTabsTime" units="ms">
+  <owner>mlamouri@chromium.org</owner>
+  <summary>
+    Records for how long multiple tabs have been audible at the same time. The

[whywhat, 2016/01/15 19:01:20]

nit: s/multiple tabs/more than one tab

[mlamouri (slow - plz ping), 2016/01/19 12:18:43]

Done.

+    concurrent tabs might change during this period. The time recorded starts
+    when two tabs are audible at the same time and stops when it is back below
+    two.
+  </summary>
+</histogram>
+
+<histogram name="Media.Audible.ConcurrentTabsWhenStarting">
+  <owner>mlamouri@chromium.org</owner>
+  <summary>
+    Records how many tabs were audible when a new tab started to be audible.
+  </summary>
+</histogram>
+
 <histogram name="Media.Audio.AutomaticGainControlMac" enum="BooleanEnabled">
   <owner>henrika@chromium.org</owner>
   <summary>