Skip migrating for default shown sections prefs.

chrome/browser/dom_ui/shown_sections_handler.cc

Index: chrome/browser/dom_ui/shown_sections_handler.cc
diff --git a/chrome/browser/dom_ui/shown_sections_handler.cc b/chrome/browser/dom_ui/shown_sections_handler.cc
index 52d3ae644ea1c2a26687a3b79840f445fda2ce0d..04d956437524437b11909ba4bb86cecee8327ea9 100644
--- a/chrome/browser/dom_ui/shown_sections_handler.cc
+++ b/chrome/browser/dom_ui/shown_sections_handler.cc
@@ -106,6 +106,12 @@ void ShownSectionsHandler::RegisterUserPrefs(PrefService* pref_service) {
 void ShownSectionsHandler::MigrateUserPrefs(PrefService* pref_service,
                                             int old_pref_version,
                                             int new_pref_version) {
+  // Nothing to migrate for default kNTPShownSections value.
+  const PrefService::Preference* shown_sections_pref =
+      pref_service->FindPreference(prefs::kNTPShownSections);
+  if (shown_sections_pref->IsDefaultValue())
+    return;
+
   bool changed = false;
   int shown_sections = pref_service->GetInteger(prefs::kNTPShownSections);