GitLab

This matches the HostResolverManager behavior where all requests get
silently cancelled if the manager is destroyed. Now if a
ContextHostResolver is destroyed, all requests created by that resolver
get silently cancelled, leaving requests from other resolvers alone.

Motivation:
For DoH, need to ensure that if a context (and its per-context resolver)
are destroyed, any requests potentially using that context for DoH are
cancelled. Previously, DoH always used the "primary" context and we took
special care to cancel all DNS requests on destruction of that context,
but DoH will soon use any context from the per-context resolvers.

Bug: 934402
Change-Id: I99d087bdd17ac8e2bd54efa4c425e63abf0568ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1549897Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#648713}