Fix appcache tests.

Adjust the wpt tests for application cache according to https://chromium-review.googlesource.com/c/chromium/src/+/1926478/20/third_party/blink/web_tests/external/wpt/fetch/metadata/appcache.tentative.https.sub.html#24.

Bug: 1024198
Change-Id: I8bf6d94fbd12e5dd5ef65aea8a317a4694057c2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1938908Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Yifan Luo <lyf@google.com>
Cr-Commit-Position: refs/heads/master@{#719936}

third_party/blink/web_tests/external/wpt/fetch/metadata/appcache.tentative.https.sub.html

@@ -21,7 +21,7 @@
         text = await response.text();
         assert_header_equals(text, expected, "Appcache resource");
       } catch (e) {
-        t.step_func(e => {
+        t.step(e => {
           if (e instanceof AssertionError) {
             throw e;
           }