Add MobileMenuDataSaverOpened to actions.xml.

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 9776 matching lines @@
   <owner>aurimas@chromium.org</owner>
   <description>User pressed 'Close all tabs' in the app menu.</description>
 </action>
 
 <action name="MobileMenuCloseTab">
   <owner>aurimas@chromium.org</owner>
   <description>User pressed 'Close tab' in the app menu.</description>
   <obsolete>This menu item was never added to the app menu.</obsolete>
 </action>
 
+<action name="MobileMenuDataSaverOpened">
+  <owner>megjablon@chromium.org</owner>
+  <description>User pressed the Data Saver item in the app menu.</description>

[tapted, 2017/04/05 12:31:51]

I didn't add quotes because the item says "{size bytes} saved since {date}". LMK
if we want something else here.

+</action>
+
 <action name="MobileMenuDirectShare">
   <owner>aurimas@chromium.org</owner>
   <description>
     User pressed the application icon next to 'Share' to share directly with the
     most recently used application.
   </description>
 </action>
 
 <action name="MobileMenuDownloadManager">
   <owner>dfalcantara@chromium.org</owner>
@@ skipping 6894 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>