Reapply "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 7c9022ec22c315bb0462b9b8e7bb52c1c459c3d9..d807b06a009028cab32af6ab4d2c1de6571406a4 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;