Switch SupportsUserData uses to use unique_ptr.

The interface taking a raw pointer is deprecated and being removed.

BUG=690937

Review-Url: https://codereview.chromium.org/2851003002
Cr-Commit-Position: refs/heads/master@{#468654}
diff --git a/components/user_prefs/user_prefs.h b/components/user_prefs/user_prefs.h
index 55e81c9..392e5dc 100644
--- a/components/user_prefs/user_prefs.h
+++ b/components/user_prefs/user_prefs.h
@@ -20,6 +20,8 @@
 // base::SupportsUserData using the UserPrefs::Set() function.
 class USER_PREFS_EXPORT UserPrefs : public base::SupportsUserData::Data {
  public:
+  ~UserPrefs() override;
+
   // Retrieves the PrefService for a given context, or null if none is attached.
   static PrefService* Get(base::SupportsUserData* context);
 
@@ -29,7 +31,6 @@
 
  private:
   explicit UserPrefs(PrefService* prefs);
-  ~UserPrefs() override;
 
   // Non-owning; owned by embedder.
   PrefService* prefs_;