Restrict kDisableSpdy pref to only disable SPDY/3.1, not HTTP/2.

HTTP/2 is now published and officially supported by many browsers and
server. No need to disable it with SPDY/3.1.

BUG=585614

Review-Url: https://codereview.chromium.org/1956573004
Cr-Commit-Position: refs/heads/master@{#395612}
2 files changed