Use URLRequestContextBuilderMojo to create the SystemURLRequestContext.

net/url_request/url_request_context_builder.cc

Index: net/url_request/url_request_context_builder.cc
diff --git a/net/url_request/url_request_context_builder.cc b/net/url_request/url_request_context_builder.cc
index 85a19f401ad5d3bb124fdcd8a7de6a78df4321ff..5d344e4cccf7b074bcb4dcc9909db54d3427abe6 100644
--- a/net/url_request/url_request_context_builder.cc
+++ b/net/url_request/url_request_context_builder.cc
@@ -336,7 +336,13 @@ std::unique_ptr<URLRequestContext> URLRequestContextBuilder::Build() {
   }
   storage->set_host_resolver(std::move(host_resolver_));
 
-  storage->set_ssl_config_service(new SSLConfigServiceDefaults);
+  if (ssl_config_service_) {
+    // This takes a raw pointer, but |storage| will hold onto a reference to the
+    // service.
+    storage->set_ssl_config_service(ssl_config_service_.get());
+  } else {
+    storage->set_ssl_config_service(new SSLConfigServiceDefaults);
+  }
 
   if (!http_auth_handler_factory_) {
     http_auth_handler_factory_ =