Integrate registry_hash_store_contents with the rest of tracked prefs.

components/user_prefs/tracked/hash_store_contents.h

Index: components/user_prefs/tracked/hash_store_contents.h
diff --git a/components/user_prefs/tracked/hash_store_contents.h b/components/user_prefs/tracked/hash_store_contents.h
index 91441c8fb91d5f904ece2420e6d8818e38fa9f17..9cc8abba09093e979833ef030a19b20e3195d05d 100644
--- a/components/user_prefs/tracked/hash_store_contents.h
+++ b/components/user_prefs/tracked/hash_store_contents.h
@@ -26,6 +26,15 @@ class HashStoreContents {
  public:
   virtual ~HashStoreContents() {}
 
+  // Returns true if this implementation of HashStoreContents can be copied via
+  // MakeCopy().
+  virtual bool IsCopyable() const = 0;
+
+  // Returns a copy of this HashStoreContents. Must only be called on
+  // lightweight implementations (which return true from IsCopyable()) and only
+  // in scenarios where a copy cannot be avoided.
+  virtual std::unique_ptr<HashStoreContents> MakeCopy() const = 0;
+
   // Returns the suffix to be appended to UMA histograms for this store type.
   // The returned value must either be an empty string or one of the values in
   // histograms.xml's TrackedPreferencesExternalValidators.