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 999559cd94f7f5c8ba14412397a1ae2110561a0c..740ff48dcd8e2ded6f4f3912ec95bfe27a59d2ac 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -18255,6 +18255,32 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.Audible.ConcurrentTabsInSession">

[Mark P, 2016/01/20 00:19:16]

Given when this histogram is emitted, it should have Maximum in the name
somewhere.

[mlamouri (slow - plz ping), 2016/01/20 11:31:52]

Done.

+  <owner>mlamouri@chromium.org</owner>
+  <summary>
+    Records how many tabs have been audible at the same time during the session.

[Mark P, 2016/01/20 00:19:16]

nit: have been -> are
(or were)

[mlamouri (slow - plz ping), 2016/01/20 11:31:52]

Done.

+    It is recording the maximum audible tab count everytime it increases. In
+    other words, a session with N concurrent audible tabs will record for [1;N].

[Mark P, 2016/01/20 00:19:16]

nit: for [1,N]. -> each number 1 through N exactly once.

[mlamouri (slow - plz ping), 2016/01/20 11:31:52]

Done.

+  </summary>
+</histogram>
+
+<histogram name="Media.Audible.ConcurrentTabsTime" units="ms">
+  <owner>mlamouri@chromium.org</owner>
+  <summary>
+    Records for how long more than one tab has been audible at the same time.

[Mark P, 2016/01/20 00:19:16]

nit: has been -> is
(or was)

[Mark P, 2016/01/20 00:19:16]

nit: omit "for"

[mlamouri (slow - plz ping), 2016/01/20 11:31:52]

Done for both.

+    The 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

[Mark P, 2016/01/20 00:19:16]

two tabs are audible -> the user goes from one to two tabs audible

[mlamouri (slow - plz ping), 2016/01/20 11:31:52]

Done with s/user/browser/

+    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>