Only send sigalgs extension in 1.2-capable ClientHellos.

BUG=https://code.google.com/p/webrtc/issues/detail?id=4223

Change-Id: I88eb036fdc6da17bc6a5179df02f35486abe9add
Reviewed-on: https://boringssl-review.googlesource.com/3030
Reviewed-by: Adam Langley <agl@google.com>
(cherry picked from commit 6ae7f072e3b220b17ca5182226de882d10080f50)