headless: Make it possible to configure an HTTP proxy

headless/lib/browser/headless_browser_impl.h

Index: headless/lib/browser/headless_browser_impl.h
diff --git a/headless/lib/browser/headless_browser_impl.h b/headless/lib/browser/headless_browser_impl.h
index 2f276c8370b39718f856fa054ca76d8914e5f3a0..86d23a90762a1cacd2493b16a82fb9018949d9a2 100644
--- a/headless/lib/browser/headless_browser_impl.h
+++ b/headless/lib/browser/headless_browser_impl.h
@@ -44,6 +44,11 @@ class HeadlessBrowserImpl : public HeadlessBrowser {
 
   const HeadlessBrowser::Options& options() const { return options_; }
 
+  // Customize the options used by this headless browser instance. Note that
+  // options which take effect before the message loop has been started (e.g.,
+  // custom message pumps) cannot be set via this method.
+  void SetOptionsForTesting(const HeadlessBrowser::Options& options);
+
  protected:
   base::Callback<void(HeadlessBrowser*)> on_start_callback_;
   HeadlessBrowser::Options options_;