Added UMA statistics for changing the active window via click or touch events.

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 2483 matching lines @@
 <action name="DeleteWordBackward">
   <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="DeleteWordForward">
   <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="Desktop_SwitchTask">
+  <owner>bruthig@google.com</owner>
+  <owner>tdanderson@google.com</owner>
+  <description>
+    Recorded when the user activates an existing task by clicking or tapping on

[tdanderson, 2015/06/04 22:15:51]

Specify this is only recorded for tasks which are windows.

[bruthig, 2015/06/05 17:28:21]

Done.

+    it in the 2-dimensional screen space.

[Alexei Svitkine (slow), 2015/06/04 18:16:04]

I find this action description pretty confusing. I think this needs some context
- i.e. that this action is ChromeOS specific and related to the task switcher.
(Also not sure why you mention "2-dimensional screen space" ... as opposed to
what?)

[tdanderson, 2015/06/04 22:15:51]

Would it be clearer if "in the 2-dimensional screen space" were omitted enirely?
Whatever change is made to the description here should be applied to the other
documentation in this CL.

[bruthig, 2015/06/05 17:28:21]

Attempted to make it better.

+  </description>
+</action>
+
 <action name="Destination_Application_Edit">
   <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="Destination_Application_Launch">
   <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 9945 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>