Add a getter for system url request context in CastBrowserContext.

chromecast/browser/cast_browser_context.cc

Index: chromecast/browser/cast_browser_context.cc
diff --git a/chromecast/browser/cast_browser_context.cc b/chromecast/browser/cast_browser_context.cc
index e86adf2a2edc436def647c4dbe0c3b9b79b3a916..5e53167015b461c1e7c2c83f47370ccb4dfe74eb 100644
--- a/chromecast/browser/cast_browser_context.cc
+++ b/chromecast/browser/cast_browser_context.cc
@@ -116,6 +116,10 @@ CastBrowserContext::GetMediaRequestContextForStoragePartition(
   return GetMediaRequestContext();
 }
 
+net::URLRequestContextGetter* CastBrowserContext::GetSystemRequestContext() {
+  return url_request_context_factory_->GetSystemGetter();
+}
+
 content::ResourceContext* CastBrowserContext::GetResourceContext() {
   return resource_context_.get();
 }