Add new method to get negotiated SSL cipher in jingle glue.

This is required to re-land CL 26009004 in webrtc (was reverted
due to compiler errors while rolling webrtc into chromium):
https://webrtc-codereview.appspot.com/26009004/

BUG=https://code.google.com/p/webrtc/issues/detail?id=3976

Review URL: https://codereview.chromium.org/896393005

Cr-Commit-Position: refs/heads/master@{#315370}

AUTHORS

@@ -227,6 +227,7 @@ Jinlong Zhai <jinlong.zhai@samsung.com>
 Jinho Bang <jinho.bang@samsung.com>
 Jinwoo Song <jinwoo7.song@samsung.com>
 Jitendra Kumar Sahoo <jitendra.ks@samsung.com>
+Joachim Bauch <mail@joachim-bauch.de>
 Joe Knoll <joe.knoll@workday.com>
 Joe Thomas <mhx348@motorola.com>
 Joel Stanley <joel@jms.id.au>

jingle/glue/channel_socket_adapter_unittest.cc

@@ -46,6 +46,7 @@ class MockTransportChannel : public cricket::TransportChannel {
   MOCK_CONST_METHOD1(GetSslRole, bool(rtc::SSLRole* role));
   MOCK_METHOD1(SetSrtpCiphers, bool(const std::vector<std::string>& ciphers));
   MOCK_METHOD1(GetSrtpCipher, bool(std::string* cipher));
+  MOCK_METHOD1(GetSslCipher, bool(std::string* cipher));
   MOCK_CONST_METHOD1(GetLocalIdentity, bool(rtc::SSLIdentity** identity));
   MOCK_CONST_METHOD1(GetRemoteCertificate,
                      bool(rtc::SSLCertificate** cert));