Remove IconController, merge it into related functions.

Most of the logic is simplified and now lives in Document::iconURLs(). The
logic for the default favicon is simplified and now lives in a static function
on IconURL.

BUG=none

Review URL: https://codereview.chromium.org/68163014

git-svn-id: svn://svn.chromium.org/blink/trunk@162086 bbb929c8-8fbe-4397-9dbb-9b2b20218538