Remove deprecated method from WebSecurityPolicy.

shouldHideReferrer() is marked to be removed from Blink when Chromium
stops using it, since the usage of this method was already removed from
Chromium, we can remove this method now.

R=abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/21702002

git-svn-id: svn://svn.chromium.org/blink/trunk@155407 bbb929c8-8fbe-4397-9dbb-9b2b20218538