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CL makes sure that SetOnResponseStartedCallback and SetOnRedirectCallback
can only be set prior to the request gets started.

This is a follow up on a request made by mmenke@ in [1].

[1] https://crrev.com/c/1142350/14/services/network/public/cpp/simple_url_loader.cc#1226

TBR=pfeldman@chromium.org,peter@chromium.org

BUG=773295

Change-Id: If937313c21df5139f1c6e21f982b228513501962
Reviewed-on: https://chromium-review.googlesource.com/c/1317949
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605417}