Don't provide a state object to initial popstate events unless the load was back/forward.

This was regressed in r149766, when we started providing a state object if one
existed for all loads.

BUG=257995
TEST=http/tests/history/popstate-reload.html

Review URL: https://chromiumcodereview.appspot.com/19429004

git-svn-id: svn://svn.chromium.org/blink/trunk@154356 bbb929c8-8fbe-4397-9dbb-9b2b20218538