Convert Class::descriptor_ from StringPiece to String (as part of image loading)

Change-Id: Iab5ffa353661a7c06ee79af1f40d399a53777174
diff --git a/src/utils.cc b/src/utils.cc
index 419eb1c..058ca83 100644
--- a/src/utils.cc
+++ b/src/utils.cc
@@ -61,9 +61,9 @@
   if (obj->GetClass() == NULL) {
     return "(raw)";
   }
-  std::string result(PrettyDescriptor(obj->GetClass()->GetDescriptor()));
+  std::string result(PrettyDescriptor(obj->GetClass()->GetDescriptor()->ToModifiedUtf8()));
   if (obj->IsClass()) {
-    result += "<" + PrettyDescriptor(obj->AsClass()->GetDescriptor()) + ">";
+    result += "<" + PrettyDescriptor(obj->AsClass()->GetDescriptor()->ToModifiedUtf8()) + ">";
   }
   return result;
 }