Add histograms to track synced pref changes.

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 3fb8a60e699592c51a226edf32ede9a24908da6b..c4d2c25a68c6f5429cc70ac0a42442ecf336a66a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -13233,6 +13233,20 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Settings.HomePageIsNewTabPage.PulledFromSync" enum="Boolean">
+  <summary>
+    The value of the home-page-is-new-tab-page pref when pulled down from sync
+    to update an out-of-sync local pref store.
+  </summary>
+</histogram>
+
+<histogram name="Settings.HomePageIsNewTabPage.PushedToSync" enum="Boolean">
+  <summary>
+    The value of the home-page-is-new-tab-page pref when pushed up to sync from
+    a change made locally.
+  </summary>
+</histogram>
+
 <histogram name="Settings.ShowHomeButton" enum="BooleanEnabled">
   <summary>
     Whether or not the home button is enabled in user preferences when a profile
@@ -13240,10 +13254,39 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Settings.ShowHomeButton.PulledFromSync" enum="BooleanEnabled">
+  <summary>
+    The enabled state of the Home button pref when pulled down from sync to
+    update an out-of-sync local pref store.
+  </summary>
+</histogram>
+
+<histogram name="Settings.ShowHomeButton.PushedToSync" enum="BooleanEnabled">
+  <summary>
+    The enabled state of the Home button pref when pushed up to sync from a
+    change made locally.
+  </summary>
+</histogram>
+
 <histogram name="Settings.StartupPageLoadSettings" enum="SessionStartupPref">
   <summary>The startup page settings when a profile is loaded.</summary>
 </histogram>
 
+<histogram name="Settings.StartupPageLoadSettings.PulledFromSync"
+    enum="SessionStartupPref">
+  <summary>
+    The startup page setting when pulled down from sync to update an out-of-sync
+    local pref store.
+  </summary>
+</histogram>
+
+<histogram name="Settings.StartupPageLoadSettings.PushedToSync"
+    enum="SessionStartupPref">
+  <summary>
+    The startup page setting when pushed up to sync from a change made locally.
+  </summary>
+</histogram>
+
 <histogram name="Settings.StartupPageLoadURLs">
   <summary>
     The number of URLs to be loaded on startup when a profile is loaded, if the