Added constants, strings, protos for app sync.

chrome/browser/sync/syncable/model_type.h

Index: chrome/browser/sync/syncable/model_type.h
diff --git a/chrome/browser/sync/syncable/model_type.h b/chrome/browser/sync/syncable/model_type.h
index 613615ec02306a7d2d60aae0f8d7ab2e7f4e186d..4295d58eba94a5ebec795e4924a9f05b6a85eca9 100644
--- a/chrome/browser/sync/syncable/model_type.h
+++ b/chrome/browser/sync/syncable/model_type.h
@@ -54,10 +54,12 @@ enum ModelType {
   THEMES,
   // A typed_url folder or a typed_url object.
   TYPED_URLS,
-  // A extension folder or a extension object.
+  // An extension folder or an extension object.
   EXTENSIONS,
   // An object represeting a set of Nigori keys.
   NIGORI,
+  // An app folder or an app object.
+  APPS,
 
   MODEL_TYPE_COUNT,
 };