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This object stores per-context state for use in host resolution. In
subsequent CLs, it will be passed through the stack instead of how raw
URLRequestContext* is passed today.

Will soon be responsible for some data currently stored per-session in
DnsSession (in this CL, implementing the DoH probe results, but not yet
using the moved results). Because that data will be both per-context
and per-session, but because we really only care about it in the most
recent "active" session, ResolveContext keeps track of the "current"
session and compares session on manipulation or retrieval of the
per-session data.

Bug: 1022059
Change-Id: Iad75fd590c929357415cce1aee2984355a7df251
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2003256
Commit-Queue: Eric Orth <ericorth@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737109}