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

headless/lib/browser/headless_browser_context_options.h

Index: headless/lib/browser/headless_browser_context_options.h
diff --git a/headless/lib/browser/headless_browser_context_options.h b/headless/lib/browser/headless_browser_context_options.h
index af9b8939c6e4c754c4daf2c35cbb42de0e0ee0fb..e0e060e067522b289a91f5f1ec163b6e92e7ac3b 100644
--- a/headless/lib/browser/headless_browser_context_options.h
+++ b/headless/lib/browser/headless_browser_context_options.h
@@ -47,6 +47,11 @@ class HeadlessBrowserContextOptions {
   // 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;
 
@@ -62,6 +67,7 @@ class HeadlessBrowserContextOptions {
   base::Optional<bool> incognito_mode_;
 
   ProtocolHandlerMap protocol_handlers_;
+  std::function<void(WebPreferences*)> override_web_preferences_callback_;
 
   DISALLOW_COPY_AND_ASSIGN(HeadlessBrowserContextOptions);
 };