SCSV support

This adds the TLS fallback SCSV token.

Bug: 17750026
(cherry-picked from commit e256f2c51cf406843a7ed0364399f79a63a085f6)

Change-Id: I9ca3e12a8a430e647bd5421fb6fee32dba360a8c
(cherry picked from commit e8612c59f7950d151e4c3b7687f540613625297f)