Make stats ping api key a header

This will give brave-core parity with iOS.

Resolves brave/brave-browser#10230

Author: kkuehlz

browser/brave_stats_updater.cc

@@ -60,8 +60,6 @@ GURL GetUpdateURL(const GURL& base_update_url,
       update_url, "dtoi", stats_updater_params.GetDateOfInstallationParam());
   update_url = net::AppendQueryParameter(
       update_url, "ref", stats_updater_params.GetReferralCodeParam());
-  update_url = net::AppendQueryParameter(
-      update_url, "api_key", brave::GetAPIKey());
   return update_url;
 }
 
@@ -198,6 +196,7 @@ void BraveStatsUpdater::SendServerPing() {
       net::LOAD_DO_NOT_SEND_COOKIES | net::LOAD_DO_NOT_SAVE_COOKIES |
       net::LOAD_BYPASS_CACHE | net::LOAD_DISABLE_CACHE |
       net::LOAD_DO_NOT_SEND_AUTH_DATA;
+  resource_request->headers.SetHeader("X-Brave-API-Key", brave::GetAPIKey());
   network::mojom::URLLoaderFactory* loader_factory =
       g_browser_process->system_network_context_manager()
           ->GetURLLoaderFactory();