Make add_cleanup step in no-inherit-type.html return undefined

This is because add_cleanup will begin accepting thenable return
values, and returning any other value will cause a harness error:
https://github.com/web-platform-tests/wpt/pull/8748#issuecomment-398954260

Change-Id: I700233ef706328c09e67c6d262f206b8f0d06da7
Reviewed-on: https://chromium-review.googlesource.com/1109513
Commit-Queue: Robert Ma <robertma@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569249}

third_party/WebKit/LayoutTests/http/tests/fetch/chromium/no-inherit-type.html

@@ -6,7 +6,7 @@
 
 promise_test(async t => {
   Object.prototype.type = 'incorrect';
-  t.add_cleanup(() => delete Object.prototype.type);
+  t.add_cleanup(() => { delete Object.prototype.type });
   const response = await fetch('no-inherit-type.html');
   const reader = response.body.getReader();
   let done;
@@ -22,7 +22,7 @@ promise_test(async t => {
     },
     set: undefined
   });
-  t.add_cleanup(() => delete Object.prototype.type);
+  t.add_cleanup(() => { delete Object.prototype.type });
   const response = await fetch('no-inherit-type.html');
   const reader = response.body.getReader();
   let done;