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This CL changes the signature of content::PreresolveUrl() so that the
method returns a net::HostResolver::Request object to a caller instead
of holding it inside of the callback state. It is done to guarantee
that there won't be hanging requests after a caller is shut down.

There are only two callers of this method:
chrome_browser_net::Predictor and predictors::PreconnectManager.
They become responsible for keeping Request objects alive.

Bug: 845648
Change-Id: Ie057cde65288341b14813d4df4d7d90a02f588b6
Reviewed-on: https://chromium-review.googlesource.com/1080818
Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564866}