Apply cross-origin auth prompt blocking to image resources only.

chrome/test/data/login/load_iframe_from_b.html

Index: chrome/test/data/login/load_iframe_from_b.html
diff --git a/chrome/test/data/login/load_iframe_from_b.html b/chrome/test/data/login/load_iframe_from_b.html
new file mode 100644
index 0000000000000000000000000000000000000000..82994c584d3ec5f31e6e3b73f347da37dc32be9c
--- /dev/null
+++ b/chrome/test/data/login/load_iframe_from_b.html
@@ -0,0 +1,14 @@
+<html>
+<body>
+This page will attempt to load an iframe requiring basic auth from the
+domain www.b.com using the same port that it loaded (since we assume www.b.com
+is an alias for this same server).
+
+<script>
+var f = document.createElement("iframe");
+f.src = "http://www.b.com:" + location.port + "/auth-basic/index.html";
+document.body.appendChild(f);
+</script>
+</body>
+</html>
+