Reland of Make ProfileIOData use URLRequestContextBuilder (patchset #1 id:1 of https://codereview.chromium.org/2986623002/ )

Reason for revert:
We're now after branch, re-landing this (Since I'll need it for a number of CLs).  I'll fix devtools in a followup CL, and merge the revert if needed.

Original issue's description:
> Revert of Make ProfileIOData use URLRequestContextBuilder (patchset #12 id:330001 of https://codereview.chromium.org/2978443002/ )
>
> Reason for revert:
> Wasn't thinking when I landed this just before branch.
>
> Original issue's description:
> > Make ProfileIOData use URLRequestContextBuilder
> >
> > BUG=734199
> >
> > Review-Url: https://codereview.chromium.org/2978443002
> > Cr-Commit-Position: refs/heads/master@{#487590}
> > Committed: https://chromium.googlesource.com/chromium/src/+/189ddf60b2f4034a2ab6efb16e22ea389b2b779f
>
> [email protected]
> # Not skipping CQ checks because original CL landed more than 1 days ago.
> BUG=734199
>
> Review-Url: https://codereview.chromium.org/2986623002
> Cr-Commit-Position: refs/heads/master@{#488562}
> Committed: https://chromium.googlesource.com/chromium/src/+/8636cbd6b9449dd374d6879b6fad0e8ed7bf61de

[email protected]
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=734199

Review-Url: https://codereview.chromium.org/2986733002
Cr-Commit-Position: refs/heads/master@{#488887}
diff --git a/chrome/browser/io_thread.cc b/chrome/browser/io_thread.cc
index 523319e..846af32 100644
--- a/chrome/browser/io_thread.cc
+++ b/chrome/browser/io_thread.cc
@@ -782,9 +782,37 @@
   return pac_https_url_stripping_enabled_.GetValue();
 }
 
-void IOThread::ConstructSystemRequestContext() {
+void IOThread::SetUpProxyConfigService(
+    net::URLRequestContextBuilderMojo* builder,
+    std::unique_ptr<net::ProxyConfigService> proxy_config_service) const {
   const base::CommandLine& command_line =
       *base::CommandLine::ForCurrentProcess();
+
+  // TODO(eroman): Figure out why this doesn't work in single-process mode.
+  // Should be possible now that a private isolate is used.
+  // http://crbug.com/474654
+  if (!command_line.HasSwitch(switches::kWinHttpProxyResolver)) {
+    if (command_line.HasSwitch(switches::kSingleProcess)) {
+      LOG(ERROR) << "Cannot use V8 Proxy resolver in single process mode.";
+    } else {
+      builder->set_mojo_proxy_resolver_factory(
+          ChromeMojoProxyResolverFactory::GetInstance());
+#if defined(OS_CHROMEOS)
+      builder->set_dhcp_fetcher_factory(
+          base::MakeUnique<chromeos::DhcpProxyScriptFetcherFactoryChromeos>());
+#endif
+    }
+  }
+
+  builder->set_pac_quick_check_enabled(WpadQuickCheckEnabled());
+  builder->set_pac_sanitize_url_policy(
+      PacHttpsUrlStrippingEnabled()
+          ? net::ProxyService::SanitizeUrlPolicy::SAFE
+          : net::ProxyService::SanitizeUrlPolicy::UNSAFE);
+  builder->set_proxy_config_service(std::move(proxy_config_service));
+}
+
+void IOThread::ConstructSystemRequestContext() {
   std::unique_ptr<net::URLRequestContextBuilderMojo> builder =
       base::MakeUnique<net::URLRequestContextBuilderMojo>();
 
@@ -823,6 +851,8 @@
 #else
   cert_verifier = net::CertVerifier::CreateDefault();
 #endif
+  const base::CommandLine& command_line =
+      *base::CommandLine::ForCurrentProcess();
   builder->SetCertVerifier(
       content::IgnoreErrorsCertVerifier::MaybeWrapCertVerifier(
           command_line, switches::kUserDataDir, std::move(cert_verifier)));
@@ -840,28 +870,8 @@
 
   builder->set_ct_verifier(std::move(ct_verifier));
 
-  // TODO(eroman): Figure out why this doesn't work in single-process mode.
-  // Should be possible now that a private isolate is used.
-  // http://crbug.com/474654
-  if (!command_line.HasSwitch(switches::kWinHttpProxyResolver)) {
-    if (command_line.HasSwitch(switches::kSingleProcess)) {
-      LOG(ERROR) << "Cannot use V8 Proxy resolver in single process mode.";
-    } else {
-      builder->set_mojo_proxy_resolver_factory(
-          ChromeMojoProxyResolverFactory::GetInstance());
-    }
-  }
-
-  builder->set_pac_quick_check_enabled(WpadQuickCheckEnabled());
-  builder->set_pac_sanitize_url_policy(
-      PacHttpsUrlStrippingEnabled()
-          ? net::ProxyService::SanitizeUrlPolicy::SAFE
-          : net::ProxyService::SanitizeUrlPolicy::UNSAFE);
-#if defined(OS_CHROMEOS)
-  builder->set_dhcp_fetcher_factory(
-      base::MakeUnique<chromeos::DhcpProxyScriptFetcherFactoryChromeos>());
-#endif
-  builder->set_proxy_config_service(std::move(system_proxy_config_service_));
+  SetUpProxyConfigService(builder.get(),
+                          std::move(system_proxy_config_service_));
 
   builder->set_http_network_session_params(session_params_);