sync: add WIFI_CREDENTIALS protobuf, ModelType, and preference

sync/protocol/sync.proto

Index: sync/protocol/sync.proto
diff --git a/sync/protocol/sync.proto b/sync/protocol/sync.proto
index 0bf405455c364d301ff85db6ef78a480db63983c..90e301ecb8725fed955035d911be7c4f28d6b3ce 100644
--- a/sync/protocol/sync.proto
+++ b/sync/protocol/sync.proto
@@ -50,6 +50,7 @@ import "synced_notification_specifics.proto";
 import "theme_specifics.proto";
 import "typed_url_specifics.proto";
 import "unique_position.proto";
+import "wifi_credential_specifics.proto";
 
 // Used for inspecting how long we spent performing operations in different
 // backends. All times must be in millis.
@@ -131,6 +132,7 @@ message EntitySpecifics {
   optional ArticleSpecifics article = 223759;
   optional AppListSpecifics app_list = 229170;
   optional EnhancedBookmarkSpecifics enhanced_bookmark = 294694;
+  optional WifiCredentialSpecifics wifi_credential = 218175;
 }
 
 message SyncEntity {