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 1821558c45557f51ad25f333c2adee6d8f9ea1cf..a77efdf2992d8794ba566944e6cdce1724e02b58 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -12932,6 +12932,22 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Settings.HomePageDomain.PulledFromSync"
+    enum="OmniboxSearchEngine">
+  <summary>
+    The id of the home page domain whenever it's pulled down from sync to update
+    an out-of-sync local pref store.
+  </summary>
+</histogram>
+
+<histogram name="Settings.HomePageDomain.PushedToSync"
+    enum="OmniboxSearchEngine">
+  <summary>
+    The id of the home page domain whenever it's pushed up to sync from a change
+    made locally.
+  </summary>
+</histogram>
+
 <histogram name="Settings.HomePageIsNewTabPage" enum="Boolean">
   <summary>
     Whether or not the home page user preference is set to the default NTP value
@@ -12939,6 +12955,20 @@ 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
@@ -12946,6 +12976,20 @@ 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.StartupPageDomains" enum="OmniboxSearchEngine">
   <summary>
     The ids of startup page domains that are specified in user preferences when
@@ -12953,10 +12997,41 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Settings.StartupPageDomains.PulledFromSync"
+    enum="OmniboxSearchEngine">
+  <summary>
+    The ids of startup page domains that are pulled down from sync to update an
+    out-of-sync local pref store.
+  </summary>
+</histogram>
+
+<histogram name="Settings.StartupPageDomains.PushedToSync"
+    enum="OmniboxSearchEngine">
+  <summary>
+    The ids of startup page domains that are 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