Update SupportsUserData uses with unique_ptr.

chrome/browser/tracing/navigation_tracing.h

Index: chrome/browser/tracing/navigation_tracing.h
diff --git a/chrome/browser/tracing/navigation_tracing.h b/chrome/browser/tracing/navigation_tracing.h
index 709379d6f8275f131f442e5f31650324efac5d32..9171373164c7032db823c46e022bdea45bbf9e43 100644
--- a/chrome/browser/tracing/navigation_tracing.h
+++ b/chrome/browser/tracing/navigation_tracing.h
@@ -18,6 +18,8 @@ class NavigationTracingObserver
     : public content::WebContentsObserver,
       public content::WebContentsUserData<NavigationTracingObserver> {
  public:
+  ~NavigationTracingObserver() override;
+
   static bool IsEnabled();
 
  private:
@@ -25,8 +27,6 @@ class NavigationTracingObserver
 
   explicit NavigationTracingObserver(content::WebContents* web_contents);
 
-  ~NavigationTracingObserver() override;
-
   // content::WebContentsObserver implementation.
   void DidStartNavigation(
       content::NavigationHandle* navigation_handle) override;