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Each individual URLRequestContext gets its own HostResolver instance.

Also, because that HostResolver becomes a dependency of the
HttpNetworkSession, constructed a new session for the media context
rather than reusing the session from the main context and built the
whole context from scratch rather than relying on the dangerous
URLRequestContext::CopyFrom(). Saved the sessions to a unique_ptr
rather than leaking it as was done previously.

Bug: 934402
Change-Id: I1c61f58406c2df66786da72146c4eb2631f6450c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529518Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#648703}