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This CL adds an implementation for PreconnectManager class together with tests.
PreconnectManager is responsible for preresolving and preconnecting to 
origins based on the input list of URLs.

Other changes:
- Modification of argument type of methods content::{Preconnect,Preresolve}Url
  from context::ResourceContext* to net::URLRequestContextGetter*.
- Adding missing LoadingPredictor::Shutdown() call in unittests + DCHECK that
  Shutdown() was actually called.

Bug: 699080
Change-Id: I5c6a303e5d22c9027da59b266a7537194b286c0b
Reviewed-on: https://chromium-review.googlesource.com/567189
Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487107}