Add support for field trial group to relax missing via header bypass logic.
This change refactors the bypass logic into a class and adds support for
a new group in the DataReductionProxyRemoveMissingViaHeaderOtherBypass
field trial. This new group relaxes the non-4xx response missing via
header bypass logic to only apply to the first response through the
proxy after a network change.
BUG=413035, 409466
Review URL: https://codereview.chromium.org/
836583002
Cr-Commit-Position: refs/heads/master@{#310698}