Add metrics for Webstore Gallery Widget app

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 12312 matching lines @@
 </action>
 
 <action name="WebsiteSettings_Opened">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <description>Please enter the description of this user action.</description>
   <obsolete>
     Superseded by the WebsiteSettings.Action histogram on 2014-10-30.
   </obsolete>
 </action>
 
+<action name="WebstoreWidgetApp.ShowDialog">
+  <owner>tbarzic@google.com</owner>
+  <description>
+    Tracks Chrome Webstore Gallery Widget app being launched.
+  </description>
+</action>
+
 <action name="Win7ASHRestart">
   <owner>cpu@chromium.org</owner>
   <owner>ananta@chromium.org</owner>
   <owner>shrikant@chromium.org</owner>
   <description>Tracks when Chrome was restarted on Win7 in Ash.</description>
 </action>
 
 <action name="Win7DesktopRestart">
   <owner>cpu@chromium.org</owner>
   <owner>ananta@chromium.org</owner>
@@ skipping 197 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>