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

headless/lib/browser/headless_browser_context_options.h

 // Copyright 2016 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.
 
 #ifndef HEADLESS_LIB_BROWSER_HEADLESS_BROWSER_CONTEXT_OPTIONS_H_
 #define HEADLESS_LIB_BROWSER_HEADLESS_BROWSER_CONTEXT_OPTIONS_H_
 
 #include <string>
 
 #include "base/files/file_path.h"
@@ skipping 29 matching lines @@
 
   // Set HeadlessBrowser::Options::incognito_mode.
   bool incognito_mode() const;
 
   // Custom network protocol handlers. These can be used to override URL
   // fetching for different network schemes.
   const ProtocolHandlerMap& protocol_handlers() const;
   // Since ProtocolHandlerMap is move-only, this method takes ownership of them.
   ProtocolHandlerMap TakeProtocolHandlers();
 
+  // Callback that is invoked to override WebPreferences for RenderViews
+  // created within this HeadlessBrowserContext.
+  const std::function<void(WebPreferences*)>&
+  override_web_preferences_callback() const;
+
  private:
   friend class HeadlessBrowserContext::Builder;
 
   explicit HeadlessBrowserContextOptions(HeadlessBrowser::Options*);
 
   HeadlessBrowser::Options* browser_options_;
 
   base::Optional<std::string> user_agent_;
   base::Optional<net::HostPortPair> proxy_server_;
   base::Optional<std::string> host_resolver_rules_;
   base::Optional<gfx::Size> window_size_;
   base::Optional<base::FilePath> user_data_dir_;
   base::Optional<bool> incognito_mode_;
 
   ProtocolHandlerMap protocol_handlers_;
+  std::function<void(WebPreferences*)> override_web_preferences_callback_;
 
   DISALLOW_COPY_AND_ASSIGN(HeadlessBrowserContextOptions);
 };
 
 }  // namespace headless
 
 #endif  // HEADLESS_LIB_BROWSER_HEADLESS_BROWSER_CONTEXT_OPTIONS_H_