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PreconnectManager is designed with restriction on the maximum number of
simultaneous dns requests. There are instances of violation of this
constraint.

Each dns job has an alternative proxy resolve job that can finish first,
cancel pending dns job and schedule a new one. However, this cancelling
is asynchronous and wouldn't have an effect if a dns job was already
scheduled in the network stack.

This CL makes the PreconnectManager wait for dns job being cancelled
before scheduling a new one.

Bug: 883806
Change-Id: I8e90ad0dce55b569a4c77b202435f70a6b776231
Reviewed-on: https://chromium-review.googlesource.com/c/1292880Reviewed-by: Benoit L <lizeb@chromium.org>
Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602409}