Add a switch to enable composited scrolling for frames and enable it on Android.

webkit/glue/webpreferences.h

Index: webkit/glue/webpreferences.h
diff --git a/webkit/glue/webpreferences.h b/webkit/glue/webpreferences.h
index f50734b6c7f60dff94399c141763e23ebcb10640..f0bc252663f374de8b7c8cacb5dc459eab93e569 100644
--- a/webkit/glue/webpreferences.h
+++ b/webkit/glue/webpreferences.h
@@ -108,6 +108,7 @@ struct WEBKIT_GLUE_EXPORT WebPreferences {
   bool show_fps_counter;
   bool accelerated_compositing_for_overflow_scroll_enabled;
   bool accelerated_compositing_for_scrollable_frames_enabled;
+  bool composited_scrolling_for_frames_enabled;
   bool show_paint_rects;
   bool render_vsync_enabled;
   bool asynchronous_spell_checking_enabled;