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Cancels pending requests causing them to never complete, same as
previous behavior for destroying the resolver. Allows stopping all
requests at the start of URLRequestContext destruction to ensure the
resolver stops working and stops any action that could depend upon
destructing context objects.

Since lots of inter-dependent context objects also have references back
to the resolver, subsequent requests are also blocked to ensure
problematic requests can not be triggered during context destruction.
Subsequently-started requests (whether created before or after the
OnShutdown() call) will always immediately fail with
ERR_CONTEXT_SHUT_DOWN. More self-consistent would be for these requests
to just hang as if they were started and cancelled, but this could
break code making assumptions about requests that should always return
a synchronous result.

Bug: 1006902
Change-Id: Idd7fb60abbdd3b21bf4e6269db58dcef6e73b6cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1819494
Commit-Queue: Eric Orth <ericorth@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701353}