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

third_party/WebKit/LayoutTests/http/tests/loading/onload/resources/onload-test.js

+jsTestIsAsync = true;
+
+var imgOnLoadCalled = false;
+var img2OnLoadCalled = false;
+var bodyOnLoadCalled = false;
+
+var img = document.createElement('img');
+var img2 = document.createElement('img');
+
+img.onload = imgOnLoad;
+img.src = url;
+document.body.appendChild(img);
+shouldBeFalse("bodyOnLoadCalled");
+shouldBeFalse("imgOnLoadCalled");
+shouldBeFalse("img.complete");
+
+function imgOnLoad() {
+  imgOnLoadCalled = true;
+  debug("img's onload");
+
+  shouldBeFalse("bodyOnLoadCalled");
+  shouldBeTrue("imgOnLoadCalled");
+  shouldBeTrue("img.complete");
+
+  img2.onload = img2OnLoad;
+  img2.src = url;
+  document.body.appendChild(img2);
+  shouldBeFalse("bodyOnLoadCalled");
+  shouldBeFalse("img2OnLoadCalled");
+  shouldBeFalse("img2.complete");
+}
+
+function img2OnLoad() {
+  img2OnLoadCalled = true;
+  debug("img2's onload");
+
+  shouldBeFalse("bodyOnLoadCalled");
+  shouldBeTrue("img2OnLoadCalled");
+  shouldBeTrue("img2.complete");
+}
+
+function bodyOnLoad() {
+  bodyOnLoadCalled = true;
+  debug("body's onload");
+  shouldBeTrue("imgOnLoadCalled");
+  shouldBeTrue("img.complete");
+  shouldBeTrue("img2OnLoadCalled");
+  shouldBeTrue("img2.complete");
+  finishJSTest();
+}