[Not committed] Make image load completion async and remove EventSender from ImageLoader

third_party/WebKit/LayoutTests/http/tests/loading/resources/preload-img-test-iframe.html

 <!--
 Running slow external script forces preloader to run for tokens below.
 FIXME: remove this after https://crbug.com/421300 is closed.
 -->
 <script type="text/javascript" src="/resources/slow-script.pl?delay=500"></script>
 
 <script>
 function onimgload() {
     if (window.internals) {
         var preloaded = internals.isPreloaded('/resources/network-simulator.php?command=increase-resource-count&path=/loading/resources/preload-test.jpg') ? "TRUE" : "FALSE";
         window.parent.postMessage('Image was preloaded: '+preloaded, '*');
     }
     window.parent.postMessage('imgload', '*');
 }
 </script>
 <img src="/resources/network-simulator.php?command=increase-resource-count&path=/loading/resources/preload-test.jpg" onerror="window.parent.postMessage('imgerror', '*')" onload="onimgload()">
+<!--
+Running slow external script forces img.onload() is called before
+ResourceFetcher::clearPreloads().
+-->
+<script type="text/javascript" src="/resources/slow-script.pl?delay=2000"></script>