Make visualViewport.scrollTop/Left readonly.

third_party/WebKit/Source/core/frame/VisualViewport.idl

 /*
  * Copyright (C) 2016 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *  * Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  *  * Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 12 matching lines @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
  // Visual Viewport API.
  // WICG proposal: https://github.com/WICG/ViewportAPI
 
 [
     RuntimeEnabled=VisualViewportAPI,
     ImplementedAs=DOMVisualViewport,
 ] interface VisualViewport : EventTarget {
-    attribute double scrollTop;
-    attribute double scrollLeft;
+    readonly attribute double scrollTop;
+    readonly attribute double scrollLeft;
 
     readonly attribute double clientWidth;
     readonly attribute double clientHeight;
 
     readonly attribute double scale;
 };