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

headless/public/headless_browser.h

Index: headless/public/headless_browser.h
diff --git a/headless/public/headless_browser.h b/headless/public/headless_browser.h
index c3043f006ad40216b6313cac3bfbd416898492e1..a1ebc5a67c772b7aa0c8898eda5ef8eeef0684d1 100644
--- a/headless/public/headless_browser.h
+++ b/headless/public/headless_browser.h
@@ -149,6 +149,14 @@ struct HeadlessBrowser::Options {
   // Run a browser context in an incognito mode. Enabled by default.
   bool incognito_mode;
 
+  // Set a callback that is invoked to override WebPreferences for RenderViews
+  // created within the HeadlessBrowser. Called whenever the WebPreferences of a
+  // RenderView change. Executed on the browser main thread.
+  //
+  // WARNING: We cannot provide any guarantees about the stability of the
+  // exposed WebPreferences API, so use with care.
+  base::Callback<void(WebPreferences*)> override_web_preferences_callback;
+
   // Reminder: when adding a new field here, do not forget to add it to
   // HeadlessBrowserContextOptions (where appropriate).
  private:
@@ -180,6 +188,8 @@ class HeadlessBrowser::Options::Builder {
   Builder& SetWindowSize(const gfx::Size& window_size);
   Builder& SetUserDataDir(const base::FilePath& user_data_dir);
   Builder& SetIncognitoMode(bool incognito_mode);
+  Builder& SetOverrideWebPreferencesCallback(
+      base::Callback<void(WebPreferences*)> callback);
 
   Options Build();