[Sync] Make SESSIONS an implicit type

sync/internal_api/public/base/model_type.h

Index: sync/internal_api/public/base/model_type.h
diff --git a/sync/internal_api/public/base/model_type.h b/sync/internal_api/public/base/model_type.h
index 16d10480cad3f706213f5825a631efe1199a801f..97ab9c21b4f37f6ea86eb8598bde4a1811662a4b 100644
--- a/sync/internal_api/public/base/model_type.h
+++ b/sync/internal_api/public/base/model_type.h
@@ -49,6 +49,11 @@ enum ModelType {
   // can be represented in the protocol using a specific Message type in the
   // EntitySpecifics protocol buffer.
   //
+  // WARNING: Modifying the order of these types or inserting a new type above
+  // these will affect numerous histograms that rely on the enum values being
+  // consistent. When adding a new type, add it to the end of the user model
+  // types section, but before the virtual types.
+  //
   // A bookmark folder or a bookmark URL object.
   BOOKMARKS,
   FIRST_USER_MODEL_TYPE = BOOKMARKS,  // Declared 2nd, for debugger prettiness.
@@ -62,7 +67,6 @@ enum ModelType {
   AUTOFILL_PROFILE,
   // An autofill folder or an autofill object.
   AUTOFILL,
-
   // A themes folder or a themes object.
   THEMES,
   // A typed_url folder or a typed_url object.
@@ -85,8 +89,20 @@ enum ModelType {
   HISTORY_DELETE_DIRECTIVES,
   // Synced push notifications.
   SYNCED_NOTIFICATIONS,
-  LAST_USER_MODEL_TYPE = SYNCED_NOTIFICATIONS,
 
+  // ---- Virtual types ----
+  // Tab sync. This is a placeholder type, so that Sessions can be implicitly
+  // enabled for history sync. It does not have a sync representation.
+  TABS,
+
+  // Virtual types are excluded from certain sync-specific events, but are
+  // considered "Real User Types".
+  FIRST_VIRTUAL_TYPE = TABS,
+  LAST_VIRTUAL_TYPE = TABS,
+
+  LAST_USER_MODEL_TYPE = TABS,
+
+  // ---- Control Types ----
   // An object representing a set of Nigori keys.
   NIGORI,
   FIRST_CONTROL_MODEL_TYPE = NIGORI,