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Motivation is that HostResolver::Resolve() is soon to be deleted and
all usage converted to CreateRequest (or to network service APIs). Need
to ensure cronet/StaleHostResolver continue working during transition.

Fully implemented StaleHostResolver::CreateRequest().
StaleHostResolver::Resolve() now implemented using CreateRequest() and
HostResolver::ResolveLegacy().

As the new API is more focused on the ResolveHostRequest object, it
made more sense to convert the StaleHostResolver::RequestImpl to be a
direct implementation, rather than using a separate Handle. Main
difference is lifetime semantics. The object is now owned by the client
instead of self-destroying on completion.  Extra orphaned requests
(allowed to complete to backfill the cache) are then detached to be
owned by StaleHostResolver instance itself as the client will likely
destroy the request after getting results.

With added weak ptr back to StaleHostResolver, it became simpler to
have RequestImpl directly read options_ and implement the stale usable
checks rather than pass in a callback.

Bug: 922699
Change-Id: Iadb48c0d574cc47ff6f79a6d313164b126c1d76d
Reviewed-on: https://chromium-review.googlesource.com/c/1452699Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Commit-Queue: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629601}