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.
+        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);
+};