Introduce ChromeZoomLevelPref, make zoom level prefs independent of profile prefs.

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 1829 matching lines @@
 <action name="Cast_Sender_CastPlayRequested">
   <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="Cast_Sender_YouTubeDeviceSelected">
   <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="Chrome_ZoomLevelPreferencesMigration">
+  <owner>wjmaclean@chromium.org</owner>
+  <description>
+    Tracks migration to per-partition zoom-level preferences.
+  </description>
+</action>
+
 <action name="ClearAuthenticationCache">
   <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="ClearBrowsingData_Autofill">
   <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 9733 matching lines @@
     apple-mobile-webapp-capable meta tag.
   </description>
 </action>
 
 <action name="webapps.AddShortcut.Bookmark">
   <owner>dfalcantara@chromium.org</owner>
   <description>Shortcut added to the homescreen for a bookmark.</description>
 </action>
 
 </actions>