[MediaNotification] Add UMA to record user clicking media notifications

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 37d05ea340390ae72400e703fefb34020bb07dcf..92f13ea67efaad0e68e5285967b7aefe2157f7dd 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -18731,6 +18731,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Launch.NotificationSource" enum="LaunchNotificationSource">

[mlamouri (slow - plz ping), 2016/04/21 12:37:26]

The name is slightly confusing: should it be more explicitly about Media? Like
Media.Notification.Click? and the source would be MediaNotificationSource?

[Zhiqiang Zhang (Slow), 2016/04/21 16:36:53]

Done.

+  <owner>mlamouri@chromium.org,avayvod@chromium.org</owner>

[mlamouri (slow - plz ping), 2016/04/21 12:37:26]

You should add your name here :)

[Zhiqiang Zhang (Slow), 2016/04/21 16:36:53]

Done.

+  <summary>
+    Records which kind of notification the user clicked to go back to Chrome.
+  </summary>
+</histogram>
+
 <histogram name="Layout.MicroSecondsPerComplexText" units="microseconds">
   <owner>benjhayden@chromium.org</owner>
   <summary>
@@ -75026,6 +75033,12 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="268959744" label="NEW_TASK | NEW_DOCUMENT"/>
 </enum>
 
+<enum name="LaunchNotificationSource" type="int">
+  <int value="0" label="Media"/>
+  <int value="1" label="Presentation"/>
+  <int value="2" label="MediaFling"/>
+</enum>
+
 <enum name="LevelDBCorruptionRestoreValue" type="int">
   <int value="0" label="Database Delete Success"/>
   <int value="1" label="Database Delete Failure"/>