Use allow attribute for delegation in wake-lock 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 screen wake lock WPTs to use the allow
attribute for delegation.

Bug: 1095641
Change-Id: Iecc6d0ce8840013c9dc570e76b1c02510eb89fef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424633
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@{#809851}

third_party/blink/web_tests/external/wpt/screen-wake-lock/wakelock-enabled-by-feature-policy.https.sub.html

@@ -28,12 +28,15 @@
     );
   }, 'Feature-Policy header {"screen-wake-lock" : ["*"]} allows same-origin iframes.');
 
+  // Set allow="screen-wake-lock" on iframe element to delegate
+  // 'screen-wake-lock' to cross origin subframe.
   async_test(t => {
     test_feature_availability(
       'navigator.wakeLock.request("screen")',
       t,
       cross_origin_src,
-      expect_feature_available_default
+      expect_feature_available_default,
+      'screen-wake-lock'
     );
   }, 'Feature-Policy header {"screen-wake-lock" : ["*"]} allows cross-origin iframes.');