Added Launcher_SwitchTask UMA user action.

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.
 
@@ skipping 7073 matching lines @@
 </action>
 
 <action name="Launcher_LaunchTask">
   <owner>bruthig@google.com</owner>
   <owner>tdanderson@google.com</owner>
   <description>
     Recorded when the user launches a new task from the shelf.
   </description>
 </action>
 
+<action name="Launcher_SwitchTask">
+  <owner>bruthig@google.com</owner>
+  <owner>tdanderson@google.com</owner>
+  <description>
+    Recorded when the user activates an existing task from the shelf.
+  </description>
+</action>
+
 <action name="LoadURL">
   <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="LoadURLFromKeyword">
   <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 5060 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>