Headless: expose WebPreferences to embedder, add --hide-scrollbars.

headless/lib/browser/headless_browser_context_options.cc

Index: headless/lib/browser/headless_browser_context_options.cc
diff --git a/headless/lib/browser/headless_browser_context_options.cc b/headless/lib/browser/headless_browser_context_options.cc
index 82e3d51ab5afb5600df42856c7c38824aa4e1281..745dd9ab6926d0466ccbd2c0c6089128ff2bf196 100644
--- a/headless/lib/browser/headless_browser_context_options.cc
+++ b/headless/lib/browser/headless_browser_context_options.cc
@@ -70,4 +70,9 @@ ProtocolHandlerMap HeadlessBrowserContextOptions::TakeProtocolHandlers() {
   return std::move(protocol_handlers_);
 }
 
+const std::function<void(WebPreferences*)>&
+    HeadlessBrowserContextOptions::override_web_preferences_callback() const {
+  return override_web_preferences_callback_;
+}
+
 }  // namespace headless