Enable user gesture requirement for autoplay experiment.

content/public/common/web_preferences.h

Index: content/public/common/web_preferences.h
diff --git a/content/public/common/web_preferences.h b/content/public/common/web_preferences.h
index 2a75e3087ba1f60a1ca79502a1c491f00d476037..54b8ed4acbe4b54197c8f720e673abdd918aa0f7 100644
--- a/content/public/common/web_preferences.h
+++ b/content/public/common/web_preferences.h
@@ -224,9 +224,12 @@ struct CONTENT_EXPORT WebPreferences {
   // Used by Android_WebView only to support legacy apps that inject script into
   // a top-level initial empty document and expect it to persist on navigation.
   bool resue_global_for_unowned_main_frame;
-  std::string autoplay_experiment_mode;
 #endif
 
+  // String that describes how media element autoplay behavior should be
+  // affected by experiment.
+  std::string autoplay_experiment_mode;
+
   // Default (used if the page or UA doesn't override these) values for page
   // scale limits. These are set directly on the WebView so there's no analogue
   // in WebSettings.