Split Document::completeURLWithOverride logic even more finely to track down crash

We're still seeing crashes in the wild in this method.

R=abarth@chromium.org
BUG=312410

Review URL: https://codereview.chromium.org/125353003

git-svn-id: svn://svn.chromium.org/blink/trunk@164532 bbb929c8-8fbe-4397-9dbb-9b2b20218538