Don't create HitTestResult just to read the URL.

This code dates back to the original upstreaming of Chrome into WebKit.
Instead lets just expose the API surface required to complete a URL from
a link and stop creating fake HitTestRequest objects.

Review URL: https://codereview.chromium.org/391243003

git-svn-id: svn://svn.chromium.org/blink/trunk@178226 bbb929c8-8fbe-4397-9dbb-9b2b20218538