DevTools : Remove 'cookiesString' output from Page.getCookies.

Fix FIXME. This CL removes 'cookiesString' output from Protocol (Page.getCookies).
It is no longer meaningful because it always has an empty string.

BUG=none.

Review URL: https://codereview.chromium.org/103803002

git-svn-id: svn://svn.chromium.org/blink/trunk@163165 bbb929c8-8fbe-4397-9dbb-9b2b20218538