Remove year+ old migration support of "session.urls_to_restore_on_startup".

chrome/browser/prefs/chrome_pref_model_associator_client.h

Index: chrome/browser/prefs/chrome_pref_model_associator_client.h
diff --git a/chrome/browser/prefs/chrome_pref_model_associator_client.h b/chrome/browser/prefs/chrome_pref_model_associator_client.h
index f0b6c943d9926a3d7e2150bea0482df8cbefe85a..4a79aabf40c8a9d4bee161f7ea3ad17eab398d72 100644
--- a/chrome/browser/prefs/chrome_pref_model_associator_client.h
+++ b/chrome/browser/prefs/chrome_pref_model_associator_client.h
@@ -31,10 +31,6 @@ class ChromePrefModelAssociatorClient
   bool IsMergeableListPreference(const std::string& pref_name) const override;
   bool IsMergeableDictionaryPreference(
       const std::string& pref_name) const override;
-  bool IsMigratedPreference(const std::string& new_pref_name,
-                            std::string* old_pref_name) const override;
-  bool IsOldMigratedPreference(const std::string& old_pref_name,
-                               std::string* new_pref_name) const override;
 
   DISALLOW_COPY_AND_ASSIGN(ChromePrefModelAssociatorClient);
 };