Add the PRF to SSL3_ENC_METHOD.

This lets us fold away the SSLv3-specific generate_master_secret. Once SSLv3
uses AEADs, others will fold away as well.

Change-Id: I27c1b75741823bc6db920d35f5dd5ce71b6fdbb3
Reviewed-on: https://boringssl-review.googlesource.com/2697
Reviewed-by: Adam Langley <agl@google.com>