Disable HTTP/2 over NPN (with OpenSSL).

* Split SSLConfig.next_proto into two members: one for ALPN, one for NPN.
* Remove HTTP/2 from NPN.
* In OpenSSL, use alpn_protos for |ALPN|, and npn_protos for |NPN|.
* In NSS, use |alpn_protos| for both.
* In NSS, disable NPN if |npn_protos| is empty.

BUG=527066

Review URL: https://codereview.chromium.org/1387363004

Cr-Commit-Position: refs/heads/master@{#355427}
15 files changed