Modify SetAlternateProtocol logging behavior.

Modify SetAlternateProtocol() behavior with respect to logging and histograms, by
ignoring alternate protocol being shadowed by |g_forced_alternate_protocol| or
being inactive due to too high probability threshold.

Also clean up HasAlternateProtocol() and GetAlternateProtocol().

BUG=392575

Review URL: https://codereview.chromium.org/858433004

Cr-Commit-Position: refs/heads/master@{#313197}
2 files changed