alexilin@: How important is net::ProxyResolutionService::TryResolveProxySynchronously() for the predictor? 

The predictor uses this call to avoid preresolving dns addresses if a proxy is enabled.

I didn't want to use this sync call in the new predictor initially but it caused a regression on perf bots. The reason is that we don't preconnect until an address is successfully resolved. Some of perf bots are offline and use proxy for testing so dns requests always fail and we can't preconnect.

We could make an async request to resolve proxy in parallel with dns resolve and go straight to the preconnect if a proxy is enabled.

alex, let me know if you can start on this since it should be unblocked now.

I'm proceeding with the plan proposed in c#2 for the new predictor, i.e. issuing an async request to resolve proxy in parallel with dns request. 

NetworkContext::LookUpProxyForURL() is the API for proxy resolution I'm going to use.

can you provide an update?

I decided to not block the network servicification on this. I'll remove the synchronous call to the ProxyResolutionProxy in my next CL and I'll add support of async resolution after that.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/248c267fc1c931ba1863d72bb1ddbd45f8680218

commit 248c267fc1c931ba1863d72bb1ddbd45f8680218
Author: Alexandr Ilin <alexilin@chromium.org>
Date: Mon Sep 10 10:40:51 2018

predictors: Request a proxy lookup in parallel with host resolve request

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}
[modify] https://crrev.com/248c267fc1c931ba1863d72bb1ddbd45f8680218/chrome/browser/BUILD.gn
[modify] https://crrev.com/248c267fc1c931ba1863d72bb1ddbd45f8680218/chrome/browser/predictors/loading_predictor_browsertest.cc
[modify] https://crrev.com/248c267fc1c931ba1863d72bb1ddbd45f8680218/chrome/browser/predictors/loading_stats_collector_unittest.cc
[modify] https://crrev.com/248c267fc1c931ba1863d72bb1ddbd45f8680218/chrome/browser/predictors/preconnect_manager.cc
[modify] https://crrev.com/248c267fc1c931ba1863d72bb1ddbd45f8680218/chrome/browser/predictors/preconnect_manager.h
[modify] https://crrev.com/248c267fc1c931ba1863d72bb1ddbd45f8680218/chrome/browser/predictors/preconnect_manager_unittest.cc
[add] https://crrev.com/248c267fc1c931ba1863d72bb1ddbd45f8680218/chrome/browser/predictors/proxy_lookup_client_impl.cc
[add] https://crrev.com/248c267fc1c931ba1863d72bb1ddbd45f8680218/chrome/browser/predictors/proxy_lookup_client_impl.h
[add] https://crrev.com/248c267fc1c931ba1863d72bb1ddbd45f8680218/chrome/test/data/predictors/proxy.pac

A synchronous proxy lookup was replaced by an asynchronous one, see c#13. Marking this as Fixed.