Switch SupportsUserData uses to use unique_ptr.

chrome/browser/extensions/chrome_extension_web_contents_observer.h

Index: chrome/browser/extensions/chrome_extension_web_contents_observer.h
diff --git a/chrome/browser/extensions/chrome_extension_web_contents_observer.h b/chrome/browser/extensions/chrome_extension_web_contents_observer.h
index 11ab88cd778902e6450d522d46974db7fa79e7d6..898dd6125316a907596e0a92b976f31358bcd284 100644
--- a/chrome/browser/extensions/chrome_extension_web_contents_observer.h
+++ b/chrome/browser/extensions/chrome_extension_web_contents_observer.h
@@ -26,12 +26,14 @@ namespace extensions {
 class ChromeExtensionWebContentsObserver
     : public ExtensionWebContentsObserver,
       public content::WebContentsUserData<ChromeExtensionWebContentsObserver> {
+ public:
+  ~ChromeExtensionWebContentsObserver() override;
+
  private:
   friend class content::WebContentsUserData<ChromeExtensionWebContentsObserver>;
 
   explicit ChromeExtensionWebContentsObserver(
       content::WebContents* web_contents);
-  ~ChromeExtensionWebContentsObserver() override;
 
   // ExtensionWebContentsObserver:
   void InitializeRenderFrame(