Make ResourceFetcher return Resources with LoadError instead of nullptrs.

third_party/WebKit/LayoutTests/http/tests/security/resources/frame-loading-via-document-write.js

 function clickAnchor()
 {
     var event = document.createEvent('MouseEvent');
     event.initEvent( 'click', true, true );
     document.getElementById('anchorLink').dispatchEvent(event);
 }
 
 function didImageLoad()
 {
     var result = document.getElementById("result");
 
     var myImg = document.getElementById("myImg");
-    if (myImg.height == 0 && myImg.width == 0) {
+    if (myImg.naturalHeight == 0 && myImg.naturalWidth == 0) {
         result.innerHTML = "Image NOT loaded.";
     } else {
         result.innerHTML = "Image loaded.";
     }
 }
 
 function lastTest()
 {
     didImageLoad();
 
     if (window.testRunner)
         testRunner.notifyDone();
 }