Introduce AlternativeServiceInfo.

* Introduce AlternativeServiceInfo and use it instead of AlternateProtocolInfo
  in HttpServerPropertiesImpl.
* Change language in method and variable names and comments to consistently talk
  about alternative service instead of alternate protocol (except for threshold
  related occurrences).
* Update HttpServerPropertiesManager to save by key "alternative_service"
  instead of "alternate_protocol", while still being able to read old
  preferences.
* Add TODO(bnc) at a couple of places.

BUG=392575

Review URL: https://codereview.chromium.org/1043973002

Cr-Commit-Position: refs/heads/master@{#322968}
11 files changed