Introduce new SPDY Version UMA histogram.
The Net.SpdyVersion UMA histogram directly tracks internal NextProto enum
values, which change, for example, when protocols are deprecated, e.g.,
https://chromium.googlesource.com/chromium/src/+/
13621d68bc4096987471ec698468a972af2fa1de%5E%21/#F9.
This CL obsoletes Net.SpdyVersion, and introduces Net.SpdyVersion2, which
uses values that are specifically generated for this purpuse, with the intent
that later versions will not change or reuse them.
BUG=412495
Review URL: https://codereview.chromium.org/
560573002
Cr-Commit-Position: refs/heads/master@{#295053}