Bug 1383683 - Fix race in wpt /wasm/wasm_local_iframe_test.html (r=bz)

MozReview-Commit-ID: AS9o85KiBMG

--HG--
extra : rebase_source : ac2748155ab400b74ca17a57ca2e17d58f67e44d

testing/web-platform/tests/wasm/wasm_local_iframe_test.html

@@ -2,16 +2,18 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script src="resources/load_wasm.js"></script>
-<iframe src="resources/frame.html" id="iframe"></iframe>
 <script>
+  function runTests(iframe) {
+      iframe = iframe.contentWindow;
       promise_test(async function() {
         var mod = await createWasmModule();
         assert_true(mod instanceof WebAssembly.Module);
         var ans = await new Promise((resolve, reject) => {
-      var iframe = document.getElementById("iframe").contentWindow;
           iframe.postMessage(mod, '*');
           window.addEventListener("message", (reply) => resolve(reply.data), false);
         });
         assert_equals(ans, 43);
       }, "send wasm module to iframe");
+  }
 </script>
+<iframe src="resources/frame.html" onload="runTests(this)"></iframe>