refine flow type of deepFreeze

Reviewed By: jstejada

Differential Revision: D7260106

fbshipit-source-id: 7bed2950bbda36c6aed690729cbfe718de60905d

Author: kassens

packages/relay-runtime/util/deepFreeze.js

@@ -17,7 +17,7 @@
  * For convenience, and for consistency with the behavior of `Object.freeze`,
  * returns the now-frozen original object.
  */
-function deepFreeze(object: Object): Object {
+function deepFreeze<T: {}>(object: T): T {
   Object.freeze(object);
   Object.getOwnPropertyNames(object).forEach(name => {
     const property = object[name];