Revert 18e9e49, only ask to close window if user has changed something

(closes facebook#1507)

Author: bhousel

modules/core/EditSystem.js

@@ -135,9 +135,11 @@ export class EditSystem extends AbstractSystem {
 
         // Setup event handlers
         window.addEventListener('beforeunload', e => {
-          e.preventDefault();
-          this.saveBackup();
-          return (e.returnValue = '');  // show browser prompt
+          if (this._history.length > 1) {  // user did something
+            e.preventDefault();
+            this.saveBackup();
+            return (e.returnValue = '');  // show browser prompt
+          }
         });
 
         window.addEventListener('unload', () => this._mutex.unlock());