Take 2 at implementing activeTab.

chrome/renderer/extensions/tab_finder.h

+// 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.
+
+#ifndef CHROME_RENDERER_EXTENSIONS_TAB_FINDER_H_
+#define CHROME_RENDERER_EXTENSIONS_TAB_FINDER_H_
+#pragma once
+
+#include "base/basictypes.h"
+#include "base/compiler_specific.h"
+#include "content/public/renderer/render_view_visitor.h"
+
+namespace content {
+class RenderView;
+}
+
+namespace extensions {
+
+// Finds the top RenderView associated with a tab ID.
+class TabFinder : public content::RenderViewVisitor {
+ public:
+  // Returns the top RenderView with |tab_id|, or NULL if none is found.
+  static content::RenderView* Find(int tab_id);
+
+ private:
+  explicit TabFinder(int tab_id);
+  virtual ~TabFinder();
+
+  // content::RenderViewVisitor implementation.
+  virtual bool Visit(content::RenderView* render_view) OVERRIDE;
+
+  int tab_id_;
+  content::RenderView* view_;
+
+  DISALLOW_COPY_AND_ASSIGN(TabFinder);
+};
+
+}  // namespace extensions
+
+#endif  // CHROME_RENDERER_EXTENSIONS_TAB_FINDER_H_