Use SystemNetworkContextManager's SharedURLLoaderFactory in Prefetch.

PrefetchServiceFactory currently uses the URLLoaderFactory from Profile.
To make it a SimpleKeyedServiceFactory, all dependencies to Profile must
be severed.

Bug: 953918
Change-Id: Icfd72e0e468a18065936d8d1286fb426fd19326b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1603386Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659615}

chrome/browser/offline_pages/prefetch/prefetch_service_factory.cc

@@ -15,6 +15,7 @@
 #include "chrome/browser/chrome_content_browser_client.h"
 #include "chrome/browser/download/download_service_factory.h"
 #include "chrome/browser/image_fetcher/image_fetcher_service_factory.h"
+#include "chrome/browser/net/system_network_context_manager.h"
 #include "chrome/browser/ntp_snippets/content_suggestions_service_factory.h"
 #include "chrome/browser/offline_pages/offline_page_model_factory.h"
 #include "chrome/browser/offline_pages/prefetch/offline_metrics_collector_impl.h"
@@ -105,9 +106,21 @@ KeyedService* PrefetchServiceFactory::BuildServiceInstanceFor(
   auto prefetch_dispatcher =
       std::make_unique<PrefetchDispatcherImpl>(profile->GetPrefs());
 
+  auto* system_network_context_manager =
+      SystemNetworkContextManager::GetInstance();
+  scoped_refptr<network::SharedURLLoaderFactory> url_loader_factory;
+  if (system_network_context_manager) {
+    url_loader_factory =
+        system_network_context_manager->GetSharedURLLoaderFactory();
+  } else {
+    // In unit_tests, NetworkService might not be available and
+    // |system_network_context_manager| would be null.
+    url_loader_factory = nullptr;
+  }
+
   auto prefetch_network_request_factory =
       std::make_unique<PrefetchNetworkRequestFactoryImpl>(
-          profile->GetURLLoaderFactory(), chrome::GetChannel(), GetUserAgent(),
+          url_loader_factory, chrome::GetChannel(), GetUserAgent(),
           profile->GetPrefs());
 
   scoped_refptr<base::SequencedTaskRunner> background_task_runner =