Merge small request making methods in DocumentThreadableLoader into the caller.

They formed separate methods since called from multiple places in past.
It no longer makes sense to have them in separate methods.

Also, pass original request to checker functions (simple request or not,
CORS enabled scheme or not).

BUG=none

Review URL: https://codereview.chromium.org/197043003

git-svn-id: svn://svn.chromium.org/blink/trunk@169095 bbb929c8-8fbe-4397-9dbb-9b2b20218538