sync: Introduce classes for per-type counters

sync/sessions/model_type_registry.h

Index: sync/sessions/model_type_registry.h
diff --git a/sync/sessions/model_type_registry.h b/sync/sessions/model_type_registry.h
index 71d304f92a1630e57807fa5aec60ec1a673bfb43..3ee36c7b3f1ce68c158ebb646f014fd31babd1bd 100644
--- a/sync/sessions/model_type_registry.h
+++ b/sync/sessions/model_type_registry.h
@@ -13,6 +13,7 @@
 #include "sync/base/sync_export.h"
 #include "sync/internal_api/public/base/model_type.h"
 #include "sync/internal_api/public/engine/model_safe_worker.h"
+#include "sync/internal_api/public/sessions/type_debug_info_observer.h"
 
 namespace syncer {
 
@@ -103,6 +104,14 @@ class SYNC_EXPORT_PRIVATE ModelTypeRegistry {
   // The set of enabled directory types.
   ModelTypeSet enabled_directory_types_;
 
+  // The set of observers of per-type debug info.
+  //
+  // Each of the DirectoryTypeDebugInfoEmitters needs such a list.  There's
+  // a lot of them, and their lifetimes are unpredictable, so it makes the
+  // book-keeping easier if we just store the list here.  That way it's
+  // guaranteed to live as long as this sync backend.
+  ObserverList<TypeDebugInfoObserver> type_debug_info_observers_;
+
   DISALLOW_COPY_AND_ASSIGN(ModelTypeRegistry);
 };