Don't depend on unregisterProperty in register-property-syntax-parsing.html

It sounds like unregisterProperty will be removed from the first version
of the P&V API, so let's not depend on it in our test.

BUG=641877

Review-Url: https://codereview.chromium.org/2369813002
Cr-Commit-Position: refs/heads/master@{#421104}
2 files changed