Force initial onpopstate event to be ignored by IronLocation.

- Sometimes the browser fires an initial popstate event and at this point the
  Router has already gotten the IronLocation.path().
- So the route runs twice on initial page load.
- Previously I was using a hacky solution that sometimes didn't work so I
  decided to be more clear about it. Only respond to onpopstate events if there
  has been at least one pushState or replaceState call.