GitLab

* Replace sparse histogram recording ALPN/NPN usage and whether there was
  overlap or fallback in case of NPN with a new histogram with much fewer
  values.  This was made possible by NPN being disabled.
* New histogram is recorded with all TLS handshakes, telling us what
  percentage of total TLS connections use HTTP/2.  (The old histogram was only
  recorded when either NPN or ALPN was used, depriving us of this piece of
  information.)
* Remove fixed values from enum NextProto; remove commented out historical
  values.  Note that git cl format puts the new enum on a single line.

BUG=526713

Review-Url: https://codereview.chromium.org/2243453002
Cr-Commit-Position: refs/heads/master@{#418664}