Use FrameTreeNode ID as frameId in extension APIs

chrome/test/data/extensions/api_test/webnavigation/crossProcessIframe/main.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.
+
+onload = function() {
+  setTimeout(function() {
+    var f = document.createElement('iframe');
+    f.onload = function() {
+      f.onload = null;
+      setTimeout(function() {
+        // I'd really like to coordinate this via postMessage / onmessage,
+        // but this doesn't seem to work with OOP extensions.

[nasko, 2015/12/09 23:06:09]

Does https://codereview.chromium.org/1495993002/ help with this comment?

[robwu, 2015/12/10 00:00:50]

Yes. Thanks!

+        f.src = f.src.replace('a.com', 'b.com');
+      }, 0);
+    };
+    f.src = 'http://a.com:' + location.search.slice(1) +
+        '/extensions/api_test/webnavigation/crossProcessIframe/frame.html';
+    document.body.appendChild(f);
+  }, 0);
+};