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() {
+  var port = location.search.slice(1);
+  setTimeout(function() {
+    var f = document.createElement('iframe');
+    window.onmessage = function(event) {
+      chrome.test.assertEq(f.contentWindow, event.source);
+      chrome.test.assertEq('http://a.com:' + port, event.origin);
+      chrome.test.assertEq('a.com: go to b.com', event.data);
+      f.src = f.src.replace('a.com', 'b.com');
+    };
+    f.src = 'http://a.com:' + port +
+        '/extensions/api_test/webnavigation/crossProcessIframe/frame.html';
+    document.body.appendChild(f);
+  }, 0);
+};