[import] Refactoring: Get rid of BlockedFromCreatingDocument state

The state can be inferred by existing state of the object.
So we can remove it and just compute the predicate.

BUG=none
TEST=none
R=dominicc,dglazkov

Review URL: https://codereview.chromium.org/135653006

git-svn-id: svn://svn.chromium.org/blink/trunk@165250 bbb929c8-8fbe-4397-9dbb-9b2b20218538