[filesapp] Add files app SWA scripts to document.head

Bug: 1113981
Change-Id: Ia32fa995fd05fbbf8baa7e710d0d435073f7effc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2585252
Auto-Submit: Noel Gordon <noel@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836507}

chromeos/components/file_manager/resources/script_loader.js

@@ -56,7 +56,7 @@ export class ScriptLoader {
       script.onload = () => resolve(this.src_);
       script.onerror = (error) => reject(error);
       script.src = this.src_;
-      document.body.append(script);
+      document.head.append(script);
     });
   }
 }