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: svn://svn.chromium.org/chrome/trunk/src@243045 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243045