Move url BUILD.gn file to main tree.

This also fixes some base errors found while url_unittests was trying to
link due to the missing entries of file_* not being present in
base/BUILD.gn.

BUG=None
TEST=gn, ninja -C out/Default url_unittests
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/124723002

git-svn-id: https://src.chromium.org/chrome/trunk/src/url@243045 4ff67af0-8c30-449e-8e8b-ad334ec8d88c