HTMLImageElement: do not use fallback content for ImageDocument

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

Index: third_party/WebKit/LayoutTests/http/tests/images/png-partial-load-as-document.html
diff --git a/third_party/WebKit/LayoutTests/http/tests/images/png-partial-load.html b/third_party/WebKit/LayoutTests/http/tests/images/png-partial-load-as-document.html
similarity index 54%
copy from third_party/WebKit/LayoutTests/http/tests/images/png-partial-load.html
copy to third_party/WebKit/LayoutTests/http/tests/images/png-partial-load-as-document.html
index 6f93e95e0d6d0e920ee7837dd959db5770f0c99a..1b37ce263bbde75186227a903f67d11ae72fdbd5 100644
--- a/third_party/WebKit/LayoutTests/http/tests/images/png-partial-load.html
+++ b/third_party/WebKit/LayoutTests/http/tests/images/png-partial-load-as-document.html
@@ -1,4 +1,9 @@
-<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();
@@ -17,13 +22,15 @@ function pngImage()
 
 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);
 }