[iOS] Set the shared URL loader factory as trusted

The system's shared URL loader factory wasn't set as trusted, probably
because no desktop shared code required it so far. With iOS CBCM,
some network requests built by shared desktop code have TrustedParams
set, and this requires the URL loader factory to be marked as trusted.

Bug: 1066495
Change-Id: I20c557ddfc99c19ffb25931ebde557eb5125e33f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346848Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Cr-Commit-Position: refs/heads/master@{#797311}

ios/chrome/browser/ios_chrome_io_thread.mm

@@ -53,6 +53,7 @@ IOSChromeIOThread::GetSharedURLLoaderFactory() {
         network::mojom::URLLoaderFactoryParams::New();
     url_loader_factory_params->process_id = network::mojom::kBrowserProcessId;
     url_loader_factory_params->is_corb_enabled = false;
+    url_loader_factory_params->is_trusted = true;
     GetSystemNetworkContext()->CreateURLLoaderFactory(
         url_loader_factory_.BindNewPipeAndPassReceiver(),
         std::move(url_loader_factory_params));