sync: add WIFI_CREDENTIALS protobuf, ModelType, and preference

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 305c385dfb2d6cd28900a1501794dd6fc42e0f62..91c28da5991dbcac86035b0a42c912cebd3c6555 100644
--- a/sync/internal_api/public/base/model_type.h
+++ b/sync/internal_api/public/base/model_type.h
@@ -112,6 +112,9 @@ enum ModelType {
   APP_LIST,
   // Enhanced bookmarks clips or folios.
   ENHANCED_BOOKMARKS,
+  // WiFi credentials. Each item contains the information for connecting to one
+  // WiFi network. This includes, e.g., network name and password.
+  WIFI_CREDENTIALS,
 
   // ---- Proxy types ----
   // Proxy types are excluded from the sync protocol, but are still considered