Switch SupportsUserData uses to use unique_ptr.

ios/chrome/browser/web/dom_altering_lock.h

Index: ios/chrome/browser/web/dom_altering_lock.h
diff --git a/ios/chrome/browser/web/dom_altering_lock.h b/ios/chrome/browser/web/dom_altering_lock.h
index f262c88429cf9772459eaccd0fbbbeb1d9076462..2ad6994de371a929d31a02e164d0c00ce0d3c229 100644
--- a/ios/chrome/browser/web/dom_altering_lock.h
+++ b/ios/chrome/browser/web/dom_altering_lock.h
@@ -41,6 +41,7 @@ typedef void (^ProceduralBlockWithBool)(BOOL);
 class DOMAlteringLock : public web::WebStateUserData<DOMAlteringLock> {
  public:
   DOMAlteringLock(web::WebState* web_state);
+  ~DOMAlteringLock() override;
 
   // This method must be called before altering the DOM of the page. This will
   // ensure that only one class tries to alter the page at a time.
@@ -57,8 +58,6 @@ class DOMAlteringLock : public web::WebStateUserData<DOMAlteringLock> {
  private:
   // DOMAltering object currently having the lock.
   base::WeakNSProtocol<id<DOMAltering>> current_dom_altering_feature_;
-
-  ~DOMAlteringLock() override;
 };
 
 #endif  // IOS_CHROME_BROWSER_WEB_DOM_ALTERING_LOCK_H_