Update SupportsUserData uses with unique_ptr.

chrome/browser/tracing/navigation_tracing.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_TRACING_NAVIGATION_TRACING_H_
 #define CHROME_BROWSER_TRACING_NAVIGATION_TRACING_H_
 
 #include "base/macros.h"
 #include "content/public/browser/background_tracing_manager.h"
 #include "content/public/browser/web_contents_observer.h"
 #include "content/public/browser/web_contents_user_data.h"
 
 namespace tracing {
 
 void SetupNavigationTracing();
 
 class NavigationTracingObserver
     : public content::WebContentsObserver,
       public content::WebContentsUserData<NavigationTracingObserver> {
  public:
+  ~NavigationTracingObserver() override;
+
   static bool IsEnabled();
 
  private:
   friend class content::WebContentsUserData<NavigationTracingObserver>;
 
   explicit NavigationTracingObserver(content::WebContents* web_contents);
 
-  ~NavigationTracingObserver() override;
-
   // content::WebContentsObserver implementation.
   void DidStartNavigation(
       content::NavigationHandle* navigation_handle) override;
 
   static content::BackgroundTracingManager::TriggerHandle
       navigation_trigger_handle_;
 
   DISALLOW_COPY_AND_ASSIGN(NavigationTracingObserver);
 };
 
 }  // namespace tracing
 
 #endif  // CHROME_BROWSER_TRACING_NAVIGATION_TRACING_H_