Speed up slow tests by reducing RSA modulus size (from 2048 --> 512).

BUG=384565,245025

Review URL: https://codereview.chromium.org/333063004

git-svn-id: svn://svn.chromium.org/blink/trunk@176145 bbb929c8-8fbe-4397-9dbb-9b2b20218538