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

headless/public/headless_browser.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "headless/public/headless_browser.h"
 
 #include <utility>
 
 #include "content/public/common/user_agent.h"
 
@@ skipping 84 matching lines @@
 Builder& Builder::SetWindowSize(const gfx::Size& window_size) {
   options_.window_size = window_size;
   return *this;
 }
 
 Builder& Builder::SetIncognitoMode(bool incognito_mode) {
   options_.incognito_mode = 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_);
 }
 
 }  // namespace headless