chore: remove pre network service classes from shell/browser/net (#19644)

* refactor: rm IOThread class

* chore: rm expose-net-observer-api.patch

* chore: rm unused shell/browser/net/ classes

* chore: mv CertVerifierClient to separate header

* chore: rm url_request_context_getter references

patches/chromium/.patches

@@ -42,7 +42,6 @@ allow_nested_error_trackers.patch
 blink_initialization_order.patch
 ssl_security_state_tab_helper.patch
 exclude-a-few-test-files-from-build.patch
-expose-net-observer-api.patch
 desktop_media_list.patch
 proxy_config_monitor.patch
 gritsettings_resource_ids.patch

patches/chromium/expose-net-observer-api.patch

@@ -1,29 +0,0 @@
-From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
-From: Jeremy Apthorp <jeremya@chromium.org>
-Date: Thu, 18 Oct 2018 17:06:56 -0700
-Subject: expose-net-observer-api.patch
-
-Expose URLRequestContextGetter::{Add,Remove}Observer.
-This patch should be removed once we switch to using the net service (cc @robo)
-
-diff --git a/net/url_request/url_request_context_getter.h b/net/url_request/url_request_context_getter.h
-index def76d3902f30ae1384ae5f629fa116b3975fa86..e5ee0bfa93f497f18bdb3ad7cd51a6726dc87f3e 100644
---- a/net/url_request/url_request_context_getter.h
-+++ b/net/url_request/url_request_context_getter.h
-@@ -79,11 +79,16 @@ class NET_EXPORT URLRequestContextGetter
-   friend class web::NetworkContextOwner;
- #endif  // defined(OS_IOS)
- 
-+  // NOTE(nornagon/robo): electron currently depends on this deprecated API
-+  // from net/, but will be removing it soon when we switch to using the
-+  // network service.
-+ public:
-   // Adds / removes an observer to watch for shutdown of |this|'s context. Must
-   // only be called on network thread. May not be called once
-   // GetURLRequestContext() starts returning nullptr.
-   void AddObserver(URLRequestContextGetterObserver* observer);
-   void RemoveObserver(URLRequestContextGetterObserver* observer);
-+ private:
- 
-   // OnDestruct is used to ensure deletion on the thread on which the request
-   // IO happens.