headless: Make it possible to configure an HTTP proxy

headless/lib/browser/headless_browser_context.cc

Index: headless/lib/browser/headless_browser_context.cc
diff --git a/headless/lib/browser/headless_browser_context.cc b/headless/lib/browser/headless_browser_context.cc
index dd4231746d79c86267e696ec841e5b1ddc01714d..6f4f191450e58e908bdcccf24c91ba49718c33ac 100644
--- a/headless/lib/browser/headless_browser_context.cc
+++ b/headless/lib/browser/headless_browser_context.cc
@@ -156,6 +156,11 @@ HeadlessBrowserContext::GetBackgroundSyncController() {
   return nullptr;
 }
 
+void HeadlessBrowserContext::SetOptionsForTesting(
+    const HeadlessBrowser::Options& options) {
+  options_ = options;
+}
+
 void HeadlessBrowserContext::SetURLRequestContextGetter(
     scoped_refptr<net::URLRequestContextGetter> url_request_context_getter) {
   resource_context_->set_url_request_context_getter(url_request_context_getter);