Use allow attribute for delegation in web serial WPT

With an upcoming change to Feature/Permissions Policy, the header
alone will no longer be sufficient to delegate permissions to use
powerful features in subframes. The iframe allow attribute must
be used in conjunction with it. The header can still be used to
block delegation.

This change updates the WebSerial WPTs to use the allow attribute
for delegation.

Bug: 1095641
Change-Id: I810cd5df06b89b0b5b2f2c160e0f9eac19dd67f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424655
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Auto-Submit: Ian Clelland <iclelland@chromium.org>
Reviewed-by: Charlie Hu <chenleihu@google.com>
Cr-Commit-Position: refs/heads/master@{#809834}

third_party/blink/web_tests/external/wpt/serial/serial-allowed-by-feature-policy.https.sub.html

@@ -27,15 +27,19 @@ async_test(t => {
       expect_feature_available_default);
 }, header + ' allows workers in same-origin iframes.');
 
+// Set allow="serial" on iframe element to delegate 'serial' to cross origin
+// subframe.
 async_test(t => {
   test_feature_availability('serial.getPorts()', t, cross_origin_src,
-      expect_feature_available_default);
+      expect_feature_available_default, 'serial');
 }, header + ' allows cross-origin iframes.');
 
+// Set allow="serial" on iframe element to delegate 'serial' to cross origin
+// subframe.
 async_test(t => {
   test_feature_availability('serial.getPorts()', t,
       cross_origin_worker_frame_src,
-      expect_feature_available_default);
+      expect_feature_available_default, 'serial');
 }, header + ' allows workers in cross-origin iframes.');
 
 fetch_tests_from_worker(new Worker(