Switch base::Value typemapping to be by value instead of by unique_ptr.

services/preferences/pref_store_manager_impl.cc

Index: services/preferences/pref_store_manager_impl.cc
diff --git a/services/preferences/pref_store_manager_impl.cc b/services/preferences/pref_store_manager_impl.cc
index 027134cffba035f91a8f36659bc31ea887a40d0c..dfa767f32fdc103d0680bd387df1c275a8cd3441 100644
--- a/services/preferences/pref_store_manager_impl.cc
+++ b/services/preferences/pref_store_manager_impl.cc
@@ -248,9 +248,9 @@ void PrefStoreManagerImpl::ConnectImpl(
     // TODO(sammc): Once non-owning registrations are supported, disallow
     // multiple owners instead of just checking for consistent defaults.
     if (defaults_->GetValue(key, &old_default))
-      DCHECK(old_default->Equals(default_value.get()));
+      DCHECK_EQ(*old_default, default_value);
     else
-      defaults_->SetDefaultValue(key, std::move(default_value));
+      defaults_->SetDefaultValue(key, default_value.CreateDeepCopy());
   }
 
   // Only connect to pref stores the client isn't already connected to.