Make GetAlternateProtocolRequestFor return AlternativeService.

Make HttpStreamFactoryImpl::GetAlternateProtocolRequestFor() return
AlternativeService instead of AlternateProtocolInfo, and rename it to
GetAlternativeServiceRequestFor.  Also change the type and name of
HttpStreamFactoryImplJob.{,other_job_}alternate_protocol_.

BUG=392576

Review URL: https://codereview.chromium.org/1011013002

Cr-Commit-Position: refs/heads/master@{#320925}
4 files changed