Add readonly to restApiService property

From TSLint prefer-readonly-rule
"Class fields should be marked as readonly if possible, to communicate
that their value is never changed and prevent later accidental
modification"

Change-Id: Ib6444a230716d2106349f8413a398ae0c1a0a5fe
diff --git a/polygerrit-ui/app/elements/change-list/gr-change-list-view/gr-change-list-view.ts b/polygerrit-ui/app/elements/change-list/gr-change-list-view/gr-change-list-view.ts
index 441d514..42741fa 100644
--- a/polygerrit-ui/app/elements/change-list/gr-change-list-view/gr-change-list-view.ts
+++ b/polygerrit-ui/app/elements/change-list/gr-change-list-view/gr-change-list-view.ts
@@ -111,7 +111,7 @@
   @property({type: String})
   _repo: string | null = null;
 
-  private restApiService = appContext.restApiService;
+  private readonly restApiService = appContext.restApiService;
 
   /** @override */
   created() {