Add UMA stat to track home button and page prefs

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 3860 matching lines @@
 </histogram>
 
 <histogram name="HistoryPage.RemoveEntryPosition">
   <summary>
     Number of entries that the deleted entry is older than in History page. Last
     bucket is any entry of that value or higher. Confirmed removal is not
     guaranteed, just an initiation of 'Remove selected items'.
   </summary>
 </histogram>
 
+<histogram name="HomePage.IsHomeButtonEnabled" enum="BooleanEnabled">

[bbudge, 2013/07/24 14:19:02]

It might be convenient to view all of these histograms on a single dashboard.
Could we name them with a common prefix, like 'Prefs' or 'Settings'? So these
two names would be:

Settings.IsHomeButtonEnabled
Settings.IsHomePageNewTabPage

Then I could name the search URL histograms:

Settings.DefaultSearchEngine
Settings.HomePageURL

etc.

+  <summary>
+    Whether or not the Home button is enabled in user preferences when a profile
+    is loaded.
+  </summary>
+</histogram>
+
+<histogram name="HomePage.IsNewTabPage" enum="Boolean">
+  <summary>
+    Whether or not the home page user preference is set to the default NTP value
+    when a profile is loaded.
+  </summary>
+</histogram>
+
 <histogram name="HttpCache.EntryLockWait" units="milliseconds">
   <summary>
     The time spent waiting for write lock on a disk cache entry.
   </summary>
 </histogram>
 
 <histogram name="HttpCache.OfflineStatus" enum="OfflineStatus">
   <summary>
     Result of a main page HttpCacheTransaction if offline mode had been enabled.
   </summary>
@@ skipping 19525 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>