Add layout tests for ImageLoader::UpdateFromElement(kUpdateForcedReload)

third_party/WebKit/LayoutTests/http/tests/cache/resources/random-cached-image.php

+<?php
+# The script name is "random-cached-image" because this is expected to return
+# different images on every (re)load to test caching, which is similar to
+# other random-cached* scripts.
+# However, this actually returns an image from a small number of predefined
+# images in sequence,
+# because it is hard to generate random (PNG/JPEG/etc.) images from scratch.
+
+require_once '../../resources/portabilityLayer.php';
+
+if (!sys_get_temp_dir()) {
+    echo "FAIL: No temp dir was returned.\n";
+    exit();
+}
+
+$id = $_GET['id'];
+if (filter_var($id, FILTER_VALIDATE_REGEXP, array("options"=>array("regexp"=>"/^[a-z0-9\-]+$/"))) === false) {
+    echo "FAIL: invalid id.\n";
+    exit();
+}
+
+$countFilename = sys_get_temp_dir() . "/random-cached-image." . $id . ".tmp";
+$count = 0;
+if (file_exists($countFilename)) {
+    $count = file_get_contents($countFilename);
+}
+$count += 1;
+file_put_contents($countFilename, $count);
+
+# Images with different dimensions.
+$imageFilenames = array(
+  '../../resources/square20.png',
+  '../../resources/square100.png',
+  '../../resources/square200.png'
+);
+
+header("Content-type: image/png");
+header("Cache-control: max-age=60000");
+header("ETag: 98765");
+
+readfile($imageFilenames[$count % count($imageFilenames)]);
+?>