So probably we want to have an instance in NetworkContext.

Yes!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ef3eae66abc67ff1334fb375e182cf5d6396582b

commit ef3eae66abc67ff1334fb375e182cf5d6396582b
Author: Takashi Toyoshima <toyoshim@chromium.org>
Date: Tue Nov 13 07:15:53 2018

OOR-CORS: Make PreflightController per-profile

Existing implementation uses a single PreflightController for all
profiles, but ideally it should be per-profile.

This patch makes the controller per-profile for NetworkService, and does
it per-factory for legacy code path due to lack of NetworkContext.

Bug:  894690 
Change-Id: Ief446431e4235bcecd68ba1f50528e721efed96a
Reviewed-on: https://chromium-review.googlesource.com/c/1331349
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607513}
[modify] https://crrev.com/ef3eae66abc67ff1334fb375e182cf5d6396582b/services/network/cors/cors_url_loader.cc
[modify] https://crrev.com/ef3eae66abc67ff1334fb375e182cf5d6396582b/services/network/cors/cors_url_loader.h
[modify] https://crrev.com/ef3eae66abc67ff1334fb375e182cf5d6396582b/services/network/cors/cors_url_loader_factory.cc
[modify] https://crrev.com/ef3eae66abc67ff1334fb375e182cf5d6396582b/services/network/cors/cors_url_loader_factory.h
[modify] https://crrev.com/ef3eae66abc67ff1334fb375e182cf5d6396582b/services/network/cors/preflight_controller.cc
[modify] https://crrev.com/ef3eae66abc67ff1334fb375e182cf5d6396582b/services/network/cors/preflight_controller.h
[modify] https://crrev.com/ef3eae66abc67ff1334fb375e182cf5d6396582b/services/network/network_context.h