HTMLImageElement: do not use fallback content for ImageDocument

third_party/WebKit/LayoutTests/http/tests/images/png-partial-load-as-document.html

-<img style="background-color: blue" >
+<iframe style="background-color: blue; width: 320px; height: 240px;"></iframe>
+<!--
+Tests that when a progressive png is loaded as document, the image is shown
+progressively during loading.
+The result image should show the dice image partially.
+-->
 <script>
 if (window.testRunner) {
     testRunner.dumpAsTextWithPixelResults();
     testRunner.waitUntilDone();
 }
 
 function loadAndStall()
 {
     return "http://127.0.0.1:8000/resources/load-and-stall.php";
 }
 
 function pngImage()
 {
     return "?name=../../../fast/images/resources/dice.png&mimeType=image%2Fpng";
 }
 
 function testDone()
 {
-    if (window.testRunner)
+    if (window.testRunner) {
+        window.stop();
         testRunner.notifyDone();
+    }
 }
 
 function runTest()
 {
-    document.querySelector("img").src = loadAndStall() + pngImage() + "&stallAt=45057&stallFor=60";
+    document.querySelector("iframe").src = loadAndStall() + pngImage() + "&stallAt=45057&stallFor=60";
     setTimeout(testDone, 500);
 }
 
 window.onload = function() {
     setTimeout(runTest, 0);
 }
 </script>