[headless] Make --hide-scrollbars affect all BrowserContexts.

headless/public/headless_browser.cc

Index: headless/public/headless_browser.cc
diff --git a/headless/public/headless_browser.cc b/headless/public/headless_browser.cc
index 2b4270bd591f333ee69a287b007c1e0030e841e7..a879e1d66d3e9fddcfd20e95a39c9a14213ceadc 100644
--- a/headless/public/headless_browser.cc
+++ b/headless/public/headless_browser.cc
@@ -102,6 +102,12 @@ Builder& Builder::SetIncognitoMode(bool incognito_mode) {
   return *this;
 }
 
+Builder& Builder::SetOverrideWebPreferencesCallback(
+    base::Callback<void(WebPreferences*)> callback) {
+  options_.override_web_preferences_callback = callback;
+  return *this;
+}
+
 Options Builder::Build() {
   return std::move(options_);
 }