Introduce V8DOMConfiguration::InstanceOrPrototype enum

This is a simple refactoring.
We should use enum instead of '1 /* on prototype */' and '0 /* on instance */'.
There is no change in behavior.

Review URL: https://codereview.chromium.org/425853006

git-svn-id: svn://svn.chromium.org/blink/trunk@179091 bbb929c8-8fbe-4397-9dbb-9b2b20218538