Mixed Content: Migrate ResourceFetcher to the static mixed content checker.

This patch begins using the static version of the mixed content checker
which was introduced in https://codereview.chromium.org/537983002/. It
also takes the opportunity to rework the error message to use the new
type information we've now passed into the checker.

This patch only touches the ResourceFetcher usage; future patches will
clean up the other points in the code where we grab a MixedContentChecker
from the loader, and eventually remove the object creation entirely.
This will make the site isolation folks happy.

BUG=411184

Review URL: https://codereview.chromium.org/561153002

git-svn-id: svn://svn.chromium.org/blink/trunk@181817 bbb929c8-8fbe-4397-9dbb-9b2b20218538