Only log HomePageIsNewTabPage if home button is enabled

tools/metrics/histograms/histograms.xml

 <!--
 Copyright 2013 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 used to generate a comprehensive list of Chrome histograms along
 with a detailed description for each histogram.
 
@@ skipping 12903 matching lines @@
 <histogram name="Search.DefaultSearchProvider" enum="OmniboxSearchEngine">
   <summary>
     The id of the default search engine that is loaded after Chrome startup. See
     src/chrome/browser/search_engines/prepopulate_engines.json for more info.
   </summary>
 </histogram>
 
 <histogram name="Settings.HomePageIsNewTabPage" enum="Boolean">
   <summary>
     Whether or not the home page user preference is set to the default NTP value
-    when a profile is loaded.
+    when a profile is loaded. This is only logged if the Home button is enabled.
   </summary>
 </histogram>
 
 <histogram name="Settings.ShowHomeButton" enum="BooleanEnabled">
   <summary>
     Whether or not the Home button is enabled in user preferences when a profile
     is loaded.
   </summary>
 </histogram>
 
@@ skipping 11114 matching lines @@
   <group name="Disabled" label="Neither extra webstore link is visible"/>
   <group name="FooterLink" label="Link in bottom right of footer"/>
   <group name="PlusIcon" label="Plus icon in apps page"/>
   <affected-histogram name="Extensions.AppLaunch"/>
   <affected-histogram name="NewTabPage.DefaultPageType"/>
 </fieldtrial>
 
 </fieldtrials>
 
 </histogram-configuration>