Add support for changing the TLS elliptic curve set.

This CL is almost identical to http://chromium-review.googlesource.com/c/611150

Bug: webrtc:8213
Change-Id: I21a8a0041a73b3171ed66b687dc47a579d45fe19
Reviewed-on: https://chromium-review.googlesource.com/653205
Commit-Queue: Diogo Real <diogor@google.com>
Reviewed-by: Peter Thatcher <pthatcher@webrtc.org>
Reviewed-by: Emad Omara <emadomara@webrtc.org>
Reviewed-by: Zeke Chin <tkchin@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Cr-Original-Commit-Position: refs/heads/master@{#19755}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7bd1f1bb5bc747af7050b22b288eebd930b9e528