Fix srcdoc attribute reset.

Currently, resetting the srcdoc attribute on an iframe in chromium fails
to clear the current content. The existing test fails since it doesn't
synchronize with the page's onload event to confirm the srcdoc content
was there before clearing the attribute and checking that the content is
cleared too.

This CL fixes both the behavior and the test, and converts the test to
be a WPT test.

Bug: 1233143
Change-Id: I006ee39e4525c7cd015f1c2483c429a93cdd0ca6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3400984
Reviewed-by: Mason Freed <masonf@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: James Maclean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/main@{#961469}

html/semantics/embedded-content/the-iframe-element/srcdoc-attribute-reset.html

@@ -0,0 +1,33 @@
+<title>Verify that clearing srcdoc resets the iframe's content.</title>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/iframe-embed-object.html#process-the-iframe-attributes">
+<link rel="author" title="James MacLean" href="mailto:wjmaclean@chromium.org">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<iframe id=myframe srcdoc='srcdoc_text'></iframe>
+<script>
+  'use strict';
+
+  async_test(function(t) {
+    window.onload = () => {
+      // Verify that the srcdoc content is loaded before we start.
+      t.step(() => {
+        assert_true(typeof myframe.contentDocument !== 'undefined',
+            'iframe has contentDocument');
+        assert_equals(myframe.contentDocument.body.innerText, 'srcdoc_text',
+            'iframe contains srcdoc content');
+      });
+
+      myframe.onload = t.step_func_done(function() {
+        assert_true(typeof myframe.contentDocument !== 'undefined',
+            'iframe has contentDocument');
+        assert_equals(myframe.contentDocument.body.innerText, '',
+            'iframe content is empty');
+      });
+
+      // Don't remove srcdoc until the initial load has completed, and the
+      // frame's onload handler is in place.
+      myframe.removeAttribute('srcdoc');
+    };
+  }, 'Verify that the frame reloads with empty body after we remove srcdoc.');
+</script>