Use base::Optional for next_protos_expected_in_ssl_config.

This will allow next_protos_expected_in_ssl_config to test for
ssl_config.alpn_protos being empty.

Bug: 801564
Change-Id: I05ee7803b7ffb5add8ef0a16f0a00c7572967041
Reviewed-on: https://chromium-review.googlesource.com/891445
Commit-Queue: Bence Béky <bnc@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532869}
3 files changed