[Chromecast] Mark system's URLLoaderFactory as trusted.

chromium/1870492 sets |trusted_params| of a ResourceRequest, which can
only be used by a trusted URLLoaderFactory.

Bug: None
Test: Manual
Change-Id: I9f589ce4161b87c4e1387c8adbfc37bc68b96cfb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900552Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Junbo Ke <juke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713193}

chromecast/browser/cast_network_contexts.cc

@@ -128,6 +128,7 @@ CastNetworkContexts::GetSystemURLLoaderFactory() {
       network::mojom::URLLoaderFactoryParams::New();
   params->process_id = network::mojom::kBrowserProcessId;
   params->is_corb_enabled = false;
+  params->is_trusted = true;
   GetSystemContext()->CreateURLLoaderFactory(
       system_url_loader_factory_.BindNewPipeAndPassReceiver(),
       std::move(params));