[Downloads] Use the initiating StoragePartition for resumption.

chrome/test/data/extensions/platform_apps/web_view/download_cookie_isolation/window.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 getRootURL(port) {
+  return new URL('http://127.0.0.1:' + port +
+      '/extensions/platform_apps/web_view/download_cookie_isolation/');
+}
+
+// Creates a new WebView using |props| and returns a Promise that resolves once
+// the 'loadstop' event is observed for the WebView.
+function createWebView(props) {
+  var webview = document.createElement('webview');
+  webview.id = props.id;
+  webview.partition = props.partition;
+  webview.addEventListener('permissionrequest', function(e) {
+    console.log("Permission request for " + e.permission);
+    if (e.permission === 'download') {
+      e.request.allow();
+    };
+  });
+  webview.addEventListener('consolemessage', function(e) {
+    console.log(props.id + " : " + e.message);
+  });
+  document.getElementById('container').appendChild(webview);
+
+  return new Promise(function(accept, reject) {
+    webview.addEventListener('loadstop', function() {
+      console.log('loadstop received with src=' + webview.src);
+      accept(true);
+    });
+    webview.src = props.url.href;
+  });
+}
+
+// Creates the WebView elements and returns a promise that resolves once all
+// webviews have finished loading.
+function createWebViews(rootUrl) {
+  var webviews = [];
+
+  return Promise.all([
+      createWebView({
+        url: new URL('guest.html#cookie=first', rootUrl),
+        id: 'first',
+        partition: 'persist:p'
+      }),
+      createWebView({
+        url: new URL('guest.html#cookie=second', rootUrl),
+        id: 'second',
+        partition: 'q'
+      })
+  ]);
+}
+
+// Called from test runner. Sends a message to the contained WebView to
+// initiate a download.
+function startDownload(id, url) {
+  console.log("Received download for " + url + " on " + id);
+  var webview = document.getElementById(id);
+  webview.contentWindow.postMessage({
+    command: 'start-download',
+    url: url
+  }, (new URL(webview.src)).origin);
+}
+
+function run() {
+  var p = new Promise(function(accept, reject) {
+    chrome.test.getConfig(function(config) {
+      accept(config);
+    });
+  }).then(function(config) {
+    return createWebViews(getRootURL(config.testServer.port));
+  }).then(function() {
+    chrome.test.sendMessage('created-webviews');
+  });
+}
+window.onload = run;