Take 2 at implementing activeTab.

chrome/renderer/extensions/tab_finder.cc

+// Copyright (c) 2012 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.
+
+#include "chrome/renderer/extensions/tab_finder.h"
+
+#include "chrome/renderer/extensions/extension_helper.h"
+#include "content/public/renderer/render_view.h"
+#include "third_party/WebKit/Source/WebKit/chromium/public/WebFrame.h"
+#include "third_party/WebKit/Source/WebKit/chromium/public/WebView.h"
+
+using content::RenderView;
+
+namespace extensions {
+
+// static
+content::RenderView* TabFinder::Find(int tab_id) {
+  TabFinder finder(tab_id);
+  RenderView::ForEach(&finder);
+  return finder.view_;
+}
+
+TabFinder::TabFinder(int tab_id) : tab_id_(tab_id), view_(NULL) {}
+
+TabFinder::~TabFinder() {}
+
+// Note: Visit returns false to terminate the iteration.
+bool TabFinder::Visit(RenderView* render_view) {
+  // Only interested in the top frame.
+  if (render_view->GetWebView()->mainFrame()->parent())
+    return true;
+
+  ExtensionHelper* helper = ExtensionHelper::Get(render_view);
+  if (helper && helper->tab_id() == tab_id_)
+    view_ = render_view;
+
+  return !view_;
+}
+
+}  // namespace extensions