Try to cut down Browser/TabContents header dependencies.

chrome/browser/browser.h

 // Copyright (c) 2010 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_BROWSER_BROWSER_H_
 #define CHROME_BROWSER_BROWSER_H_
 
 #include <map>
 #include <set>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/gtest_prod_util.h"
 #include "base/scoped_ptr.h"
-#include "base/task.h"
 #include "chrome/browser/command_updater.h"
 #include "chrome/browser/pref_member.h"
 #include "chrome/browser/sessions/session_id.h"
 #include "chrome/browser/sessions/tab_restore_service.h"
 #include "chrome/browser/shell_dialogs.h"
 #include "chrome/browser/tabs/tab_strip_model.h"
 #include "chrome/browser/tab_contents/page_navigator.h"
 #include "chrome/browser/tab_contents/tab_contents_delegate.h"
 #include "chrome/browser/toolbar_model.h"
 #include "chrome/common/extensions/extension.h"
@@ skipping 1019 matching lines @@
   mutable BooleanPrefMember use_vertical_tabs_;
 
   // The profile's tab restore service. The service is owned by the profile,
   // and we install ourselves as an observer.
   TabRestoreService* tab_restore_service_;
 
   DISALLOW_COPY_AND_ASSIGN(Browser);
 };
 
 #endif  // CHROME_BROWSER_BROWSER_H_