Use network::ResourceRequestInfo::proxy_server instead of was_fetched_via_proxy

... and remove the later.

This is a follow up of mmenke's request in [1].

[1] https://crrev.com/c/1251642/3/services/network/url_loader.cc#70

TBR=nparker@chromium (components/safe_browsing/browser/threat_details_cache.cc)
TBR=jam@chromium.org (content/browser/loader/resource_loader.cc)

BUG=773295

Change-Id: I932315bec7d1790dbc8726eeb1decce6e76172b1
Reviewed-on: https://chromium-review.googlesource.com/c/1320769
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606864}
7 files changed