[Downloads] Use the initiating StoragePartition for resumption.

chrome/test/data/extensions/platform_apps/web_view/download_cookie_isolation/guest.js

+// Copyright 2016 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.
+
+function setupMessageHandler() {
+  window.onmessage = function(e) {
+    if (e.data.command === 'start-download') {
+      console.log("Start Download received for URL " + e.data.url);
+      var anchor = document.createElement('a');
+      anchor.href = e.data.url;
+      anchor.download = 'foo';
+      document.getElementById('container').appendChild(anchor);
+      anchor.click();
+    }
+  };
+}
+
+function setCookiesBasedOnURLFragment() {
+  var cookieValue = window.location.hash.substr(1);
+  cookieValue += ";path=/";
+  document.cookie = cookieValue;
+  console.log("Setting cookie: " + cookieValue);
+}
+
+window.onload = function() {
+  setCookiesBasedOnURLFragment();
+  setupMessageHandler();
+}