Remove implementation of Panels on OSes other than ChromeOS.

chrome/browser/task_manager/providers/web_contents/panel_tag.h

-// 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.
-
-#ifndef CHROME_BROWSER_TASK_MANAGER_PROVIDERS_WEB_CONTENTS_PANEL_TAG_H_
-#define CHROME_BROWSER_TASK_MANAGER_PROVIDERS_WEB_CONTENTS_PANEL_TAG_H_
-
-#include "base/macros.h"
-#include "chrome/browser/task_manager/providers/web_contents/panel_task.h"
-#include "chrome/browser/task_manager/providers/web_contents/web_contents_tag.h"
-
-namespace task_manager {
-
-// Defines a concrete UserData type for WebContents owned by the PanelManager.
-class PanelTag : public WebContentsTag {
- public:
-  // task_manager::WebContentsTag:
-  PanelTask* CreateTask() const override;
-
- private:
-  friend class WebContentsTags;
-
-  PanelTag(content::WebContents* web_contents, Panel* panel);
-  ~PanelTag() override;
-
-  Panel* panel_;
-
-  DISALLOW_COPY_AND_ASSIGN(PanelTag);
-};
-
-}  // namespace task_manager
-
-#endif  // CHROME_BROWSER_TASK_MANAGER_PROVIDERS_WEB_CONTENTS_PANEL_TAG_H_