headless: Make it possible to configure an HTTP proxy

headless/lib/browser/headless_browser_context.h

Index: headless/lib/browser/headless_browser_context.h
diff --git a/headless/lib/browser/headless_browser_context.h b/headless/lib/browser/headless_browser_context.h
index 41b72116522aba81c453ffc1ef224b0e0d3c4819..15e9436fc1e05895dc84629ee87f69638b256842 100644
--- a/headless/lib/browser/headless_browser_context.h
+++ b/headless/lib/browser/headless_browser_context.h
@@ -44,6 +44,7 @@ class HeadlessBrowserContext : public content::BrowserContext {
   content::BackgroundSyncController* GetBackgroundSyncController() override;
 
   const HeadlessBrowser::Options& options() const { return options_; }
+  void SetOptionsForTesting(const HeadlessBrowser::Options& options);
 
   // Configure the URL request context getter to be used for serving URL
   // requests in this browser instance.