Add a declarative way in c/b/s/sync_prefs.cc to say which data types have prefs

chrome/browser/sync/sync_prefs.h

Index: chrome/browser/sync/sync_prefs.h
diff --git a/chrome/browser/sync/sync_prefs.h b/chrome/browser/sync/sync_prefs.h
index bb67b1c3812c26210a06d427d25519a84bcbf862..a599d15689b27c6f94853e933b9bd3255156caa3 100644
--- a/chrome/browser/sync/sync_prefs.h
+++ b/chrome/browser/sync/sync_prefs.h
@@ -118,6 +118,7 @@ class SyncPrefs : public base::SupportsWeakPtr<SyncPrefs>,
   syncable::ModelTypeSet GetAcknowledgeSyncedTypesForTest() const;
 
  private:
+  void RegisterPrefGroups();
   void RegisterPreferences();
 
   void RegisterDataTypePreferredPref(
@@ -125,6 +126,12 @@ class SyncPrefs : public base::SupportsWeakPtr<SyncPrefs>,
   bool GetDataTypePreferred(syncable::ModelType type) const;
   void SetDataTypePreferred(syncable::ModelType type, bool is_preferred);
 
+  // Returns a ModelTypeSet based on |types| expanded to include pref groups
+  // (see |pref_groups_|), but as a subset of |registered_types|.
+  syncable::ModelTypeSet ResolvePrefGroups(
+      syncable::ModelTypeSet registered_types,
+      syncable::ModelTypeSet types) const;
+
   base::NonThreadSafe non_thread_safe_;
 
   // May be NULL.
@@ -136,6 +143,16 @@ class SyncPrefs : public base::SupportsWeakPtr<SyncPrefs>,
   // configuration management.
   BooleanPrefMember pref_sync_managed_;
 
+  // Groups of prefs that always have the same value as a "master" pref.
+  // For example, the APPS group has {APP_NOTIFICATIONS, APP_SETTINGS}
+  // (as well as APPS, but that is implied), so
+  //   pref_groups_[syncable::APPS] =       { syncable::APP_NOTIFICATIONS,
+  //                                          syncable::APP_SETTINGS }
+  //   pref_groups_[syncable::EXTENSIONS] = { syncable::EXTENSION_SETTINGS }
+  // etc.
+  typedef std::map<syncable::ModelType, syncable::ModelTypeSet> PrefGroupsMap;
+  PrefGroupsMap pref_groups_;
+
   DISALLOW_COPY_AND_ASSIGN(SyncPrefs);
 };