Add metrics regarding concurrent audible tabs in Chromium.

tools/metrics/actions/actions.xml

Index: tools/metrics/actions/actions.xml
diff --git a/tools/metrics/actions/actions.xml b/tools/metrics/actions/actions.xml
index 0606a32f5958d65251a4c04e65060b4854813955..88354e28fd67852e2218d12289d6eef2f026152e 100644
--- a/tools/metrics/actions/actions.xml
+++ b/tools/metrics/actions/actions.xml
@@ -7718,6 +7718,16 @@ should be able to be added at any place in this file.
   <description>Please enter the description of this user action.</description>
 </action>
 
+<action name="Media.Audible.AddTab">
+  <owner>mlamouri@chromium.org</owner>
+  <description>A new tab became audible.</description>

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

omit "new"; it's slightly confusing and unnecessary.
(Unless you are only emitting when a tab becomes audible for the first time. 
E.g., if a tab X starts playing sound, stops playing sound, then starts again. 
In that case, I expect you to record two user actions.  If you only record one,
we need to revise this description.)

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

Good catch, I didn't mean to record only once per tab but when a tab becomes
audible. Removed "new".

+</action>
+
+<action name="Media.Audible.RemoveTab">
+  <owner>mlamouri@chromium.org</owner>
+  <description>A tab is no longer audible.</description>
+</action>
+
 <action name="MediaContextMenu_Controls">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <description>Please enter the description of this user action.</description>