Add a partial load test for an animated GIF image

third_party/WebKit/LayoutTests/http/tests/images/gif-animated-partial-load.html

+<img style="background: green" >
+<script>
+if (window.testRunner) {
+  testRunner.dumpAsTextWithPixelResults();
+  testRunner.waitUntilDone();
+}
+
+function loadAndStall()
+{
+  return "http://127.0.0.1:8000/resources/load-and-stall.php";
+}
+
+function gifImage()
+{
+  return "?name=../../../images/resources/count-down-color-test.gif&mimeType=image%2Fgif";
+}
+
+function testDone()
+{
+  window.testRunner && testRunner.notifyDone();
+}
+
+function runTest()
+{
+  document.querySelector("img").src = loadAndStall() + gifImage() + "&stallAt=4952&stallFor=60";
+  setTimeout(testDone, 1500);
+}
+
+window.onload = function() {
+  setTimeout(runTest, 0);
+}
+</script>