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This CL adds an asynchronous proxy lookup request to the PreconnectManager
after the synchronous request was removed in https://crrev.com/c/1165550.
The idea is that we don't need to wait for a host to be resolved before a
preconnect if a proxy is enabled.

Since we don't have a synchronous API for proxy lookup anymore, we issue both
host and proxy requests in parallel. Then we wait until either one of the
requests completes with success or both requests fail.

Bug: 838763
Change-Id: Ib3fa47884ba82d1fc12c62f7fd24dd21c279e9c5
Reviewed-on: https://chromium-review.googlesource.com/1202222
Commit-Queue: Alexandr Ilin <alexilin@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589877}