Simplify WebKit::elementHasLegalLinkAttribute.

BUG=none
TEST=none; this is just a refactoring.

Review URL: https://codereview.chromium.org/14280002

git-svn-id: svn://svn.chromium.org/blink/trunk@148431 bbb929c8-8fbe-4397-9dbb-9b2b20218538