Factor out the last half of DocumentThreadableLoader ctor

We have the same code for ServiceWorker fallback handling. Factor it out
into dispatchInitialRequest(). Also, add ASSERTs to places where
m_fallbackRequestForServiceWorker is unexpected.

R=horo,kouhei
BUG=none

Review URL: https://codereview.chromium.org/678693002

git-svn-id: svn://svn.chromium.org/blink/trunk@184424 bbb929c8-8fbe-4397-9dbb-9b2b20218538