Call SrtpTransport::EnableExternalAuth when enabling SDES.
Call SrtpTransport::EnableExternalAuth when enabling SDES.

When enabling SDES in BaseChannel, SrtpTransport::EnableExternalAuth
should be called if the external authenication is enabled which is
not covered by the current test set.

TBR=pthatcher@webrtc.org

Bug: webrtc:7013
Change-Id: Ibbf458516a521a488e8e3bb4a5a29fca70a627f5
Reviewed-on: https://webrtc-review.googlesource.com/27761
Reviewed-by: Steve Anton <93f73658da16553b94c040887a439a155812e54e@webrtc.org>
Commit-Queue: Zhi Huang <64514bc03b49e67cfb75da60b665f8d8aefa50ca@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#20960}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc