Prepare for per-context resolvers with new creation methods

Rewrote the standard HostResolver::Create...Resolver() methods. There
are now two general methods, one that takes a shared HostResolverManager
pointer, and one that creates a "standalone" resolver, matching current
behavior. Took the opportunity to combine the behaviors of CreateDefault
and CreateSystem since C++11 default args are a cleaner way to set
default options (at least for static methods like these).

Added TODOs to all the non-test and non-standalone tools that will need
to be converted to calling the creation with shared manager, but such
conversions are left to subsequent CLs.

TBR=seantopping@chromium.org,eugenebut@chromium.org,dimich@chromium.org,mmenke@chromium.org

Bug: 934402
Change-Id: I126b61db55b61ebd75b7b6f526b31ef0f0a02404
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1531391
Commit-Queue: Eric Orth <ericorth@chromium.org>
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643385}
24 files changed