Do not allow the server certificate to change in a renegotiation.

      R=agl@chromium.org,rsleevi@chromium.org
BUG=306959
TEST=none

Review URL: https://codereview.chromium.org/27266002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229611 0039d316-1c4b-4281-b951-d872f2087c98