Port downloads.ItemView to a Polymer component.

chrome/browser/resources/md_downloads/icon_loader.js

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+cr.define('downloads', function() {
+  // TODO(dbeam): does this need to be a function + @constructor?
+  var IconLoader = {};
+
+  /** @private {Array<{img: HTMLImageElement, url: string}>} */
+  IconLoader.iconsToLoad_ = [];
+
+  /**
+   * Load the provided |url| into |img.src| after appending ?scale=.
+   * @param {!HTMLImageElement} img An <img> to show the loaded image in.
+   * @param {string} url A remote image URL to load.
+   */
+  IconLoader.loadScaledIcon = function(img, url) {
+    var scale = '?scale=' + window.devicePixelRatio + 'x';
+    IconLoader.iconsToLoad_.push({img: img, url: url + scale});
+    IconLoader.loadNextIcon_();
+  };
+
+  /** @private */
+  IconLoader.loadNextIcon_ = function() {
+    if (IconLoader.isIconLoading_)
+      return;
+
+    IconLoader.isIconLoading_ = true;
+
+    while (IconLoader.iconsToLoad_.length) {
+      var request = IconLoader.iconsToLoad_.shift();
+      var img = request.img;
+
+      if (img.src == request.url)
+        continue;
+
+      img.onabort = img.onerror = img.onload = function() {
+        IconLoader.isIconLoading_ = false;
+        IconLoader.loadNextIcon_();
+      };
+
+      img.src = request.url;
+      return;
+    }
+
+    // If we reached here, there's no more work to do.
+    IconLoader.isIconLoading_ = false;
+  };
+
+  return {IconLoader: IconLoader};
+});