GitLab

Introduce struct AlternativeService and use it in
BrokenAlternateProtol{List,Map} instead of BrokenAlternateProtocolEntry.  This
is the first step towards making an alternative service broken not with respect
to an origin, but by itself.

BUG=392576

Review URL: https://codereview.chromium.org/987813002

Cr-Commit-Position: refs/heads/master@{#319680}