Blink implementation of PushManager#hasPermission()


Extra Layout tests for these new methods will depend on a change in content_shell, so they will follow in a future patch

BUG=401424

Review URL: https://codereview.chromium.org/658723003

git-svn-id: svn://svn.chromium.org/blink/trunk@184638 bbb929c8-8fbe-4397-9dbb-9b2b20218538