Change next_proto member type.
Change type of next_proto members of HttpNetworkTransaction and SSLConfig from
vector<string> to NextProtoVector. This offers better type safety and clarity.
Also, when SSLClientConfig needs to remove HTTP/2 protocols due to inadequate
security, it would feel silly having to convert strings back to NextProto. It
just makes sense to postpone generating strings until very close to the wire.
BUG=436835
Review URL: https://codereview.chromium.org/
794563003
Cr-Commit-Position: refs/heads/master@{#307894}