headless: Make it possible to configure an HTTP proxy

headless/lib/browser/headless_browser_impl.cc

Index: headless/lib/browser/headless_browser_impl.cc
diff --git a/headless/lib/browser/headless_browser_impl.cc b/headless/lib/browser/headless_browser_impl.cc
index 6bc2b5dcf96789962769d0aa22dacd3d8fb6fc4f..90730bbb3a78a52dd1a3a87d574eb22a6dee37af 100644
--- a/headless/lib/browser/headless_browser_impl.cc
+++ b/headless/lib/browser/headless_browser_impl.cc
@@ -74,6 +74,12 @@ void HeadlessBrowserImpl::RunOnStartCallback() {
   on_start_callback_ = base::Callback<void(HeadlessBrowser*)>();
 }
 
+void HeadlessBrowserImpl::SetOptionsForTesting(
+    const HeadlessBrowser::Options& options) {
+  options_ = options;
+  browser_context()->SetOptionsForTesting(options);
+}
+
 int HeadlessBrowserMain(
     const HeadlessBrowser::Options& options,
     const base::Callback<void(HeadlessBrowser*)>& on_browser_start_callback) {