Specify an ECDH group for ECDHE.

By default, OpenSSL cannot negotiate ECDHE cipher suites as a server because it
doesn't know what curve to use.

BUG=chromium:406458
TEST=Download Firefox nightly build from 2014-08-12.
  https://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/2014-08-12-mozilla-central-debug/
  Point Firefox to https://apprtc.appspot.com
  Point Chrome on Android to the URL Firefox redirects to (it'll say ?r=NUMBERS at the end)
  After tapping through various permissions prompts on either side, the call goes through.

R=agl@chromium.org, henrike@webrtc.org, jiayl@webrtc.org, juberti@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/18269004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7002 4adac7df-926f-26a2-2b94-8c16560cd09d