sample_muxer: Replace std::tmpnam() with libwebm::GetTempFileName().
sample_muxer: Replace std::tmpnam() with libwebm::GetTempFileName().

Change-Id: I3f26cb1ff286613de02c6017fa1246ca5c32d1e9

webmproject/libwebm,webmproject/libwebm,webmproject/libwebm