Use WebRTC API to convert byteorder in srtpfilter.
This CL uses WebRTC API to convert 64bit from big-endian to host-endian,
so the internal "be64_to_cpu" of libsrtp is not used. The code path of
"be64_to_cpu" in newer versions of libsrtp depends on compile-time
defines that are not available in WebRTC.
Review URL: https://webrtc-codereview.appspot.com/46749004
(cherry picked from commit 4b3c0d6f34c379c2a06f654b62403876b20180c6)