[Media Router] Add P1 UMA metrics for Media Router.

tools/metrics/actions/actions.xml

 <!--
 Copyright 2014 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is a comprehensive list of Chrome user actions along with the owner
 and description for each user action. A user action should be tied to an actual
 action taken by a user. It should be logged immediatly after a user takes the
@@ skipping 7685 matching lines @@
   </description>
 </action>
 
 <action name="MediaIndicatorButton_Unmute">
   <owner>miu@chromium.org</owner>
   <description>
     User clicked the tab media indicator button to unmute the tab.
   </description>
 </action>
 
+<action name="MediaRouter_Icon_Click">
+  <owner>apacible@chromium.org</owner>
+  <description>
+    User clicked the MediaRouterAction icon, also known as Cast icon, to open
+    the Media Router dialog.
+  </description>
+</action>
+
 <action name="Menu_Take_Screenshot">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <description>Please enter the description of this user action.</description>
 </action>
 
 <action name="MinButton_Clk">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <description>Please enter the description of this user action.</description>
 </action>
 
@@ skipping 5339 matching lines @@
 <action name="webapps.AddShortcut.Manifest">
   <owner>mlamouri@chromium.org</owner>
   <description>
     Shortcut added to the homescreen with a valid Manifest. This action will be
     recorded in addition to AppShortcut, AppShortcutApple and Bookmark actions
     to keep a trend of whether the webapp also add meta tags.
   </description>
 </action>
 
 </actions>