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This is a reland of f4240b29

This fixes two issues:

- A real UAF in ServiceResolverImplMac::OnResolveComplete, where the
  callback can delete |this|, so StopResolving() is now called before
  the callback.
- A test-only UAF caused by the background thread outliving the object.
  The thread is now flushed and joined. The -stop methods for both
  NetServiceBrowser and NetServiceResolver are now called in their
  respective -deallocs, in case the Stop posted task does not run.

Original change's description:
> mac: Simplify the local_discovery ServiceWatcher and ServiceResolver.
>
> This removes the inner Container classes by moving the logic into the
> ObjC classes that already exist.
>
> Bug: 1072841
> Change-Id: If22d2d90ce3235ce160a0b740337fd71353a7ef7
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2243995
> Commit-Queue: Robert Sesek <rsesek@chromium.org>
> Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#778341}

Bug: 1072841
Change-Id: Ice53eb92d6c73c75ad6f276b301eaa1841536c8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2248076Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779350}