Split the constructor of ThreadableLoader into two methods (ctor and start())

LayoutTests/http/tests/security/mixedContent/resources/frame-with-insecure-async-xhr-post.html

Index: LayoutTests/http/tests/security/mixedContent/resources/frame-with-insecure-async-xhr-post.html
diff --git a/LayoutTests/http/tests/security/mixedContent/resources/frame-with-insecure-async-xhr-post.html b/LayoutTests/http/tests/security/mixedContent/resources/frame-with-insecure-async-xhr-post.html
new file mode 100644
index 0000000000000000000000000000000000000000..6d78fa2ec75541281675f60e0ec43b72218f6e53
--- /dev/null
+++ b/LayoutTests/http/tests/security/mixedContent/resources/frame-with-insecure-async-xhr-post.html
@@ -0,0 +1,13 @@
+<script>
+var xhr = new XMLHttpRequest();
+xhr.onload = function()
+{
+    window.opener.postMessage('FAIL: XHR succeeded unexpectedly', '*');
+}
+xhr.onerror = function()
+{
+    window.opener.postMessage('DONE', '*');
+};
+xhr.open("POST", "http://127.0.0.1:8000/xmlhttprequest/resources/access-control-allow-lists.php?origin=*", true);
+xhr.send("foo");
+</script>