Add ASSERTs to DocumentThreadableLoader's methods which are only for async mode

We call setResource() of ResourceOwner only in async mode.

R=mkwst
BUG=none

Review URL: https://codereview.chromium.org/680483004

git-svn-id: svn://svn.chromium.org/blink/trunk@184951 bbb929c8-8fbe-4397-9dbb-9b2b20218538