Small XSSAuditor cleanup.

Avoid keeping a string for longer than necessary, and avoid the
possibility of checking for it more than once. Instead, check once
and keep that result.

R=abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19446002

git-svn-id: svn://svn.chromium.org/blink/trunk@154354 bbb929c8-8fbe-4397-9dbb-9b2b20218538