Add UMA stat to track home button and page prefs

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 897e120c7e2a02280e3cfc8d599edff57e305ea5..21c2427ca496b93489739c858af1a813c1702b68 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -3878,6 +3878,20 @@ other types of suffix sets.
   </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.