Fix wrong endianness in RTCP tag calculation for AEAD/GCM.

The sequence number was used as little-endian when the RTCP tag was
calculated which is wrong according to RFC 7714.

This was fixed upstream in https://github.com/cisco/libsrtp/commit/da852b0c253f6494bd70ae254808669acea9ea63
See https://github.com/cisco/libsrtp/issues/137 for details.

BUG=webrtc:5222
R=pthatcher@webrtc.org

Review URL: https://codereview.chromium.org/1607613003 .
1 file changed