Bug 1611371 [wpt PR 21405] - Fix `sandbox-disallow-scripts-via-unsand…

…boxed-popup.tentative.html`., a=testonly

Automatic update from web-platform-tests
Fix `sandbox-disallow-scripts-via-unsandboxed-popup.tentative.html`. (#21405)

whatwg/html#4078 changed `opener` semantics to require
explicit transfer to `_blank` targets via `rel=opener`. This patch brings this test into
line with that behavior.
--

wpt-commits: ccb23a9c120c02390a704ea40bb9425ef74ea03b
wpt-pr: 21405

testing/web-platform/tests/html/browsers/sandboxing/sandbox-disallow-scripts-via-unsandboxed-popup.tentative.html

@@ -6,9 +6,9 @@
     async_test(t => {
       let i = document.createElement('iframe');
       i.sandbox = "allow-same-origin allow-popups allow-popups-to-escape-sandbox";
-      i.srcdoc = `<a target='_blank'
+      i.srcdoc = `<a target='_blank' rel='opener'
                      href="javascript:window.opener.top.postMessage('FAIL', '*');">Click me!</a>
-                  <a target='_blank'
+                  <a target='_blank' rel='opener'
                      href="./resources/post-done-to-opener.html">Click me next!</a>`;
 
       i.onload = _ => {