Have tests verify clientX/Y and pageX/Y properties in drag-and-drop DOM events.

chrome/test/data/drag_and_drop/event_monitoring.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /* Reports an event to DragAndDropBrowserTest and  DOMDragEventWaiter */
 window.reportDragAndDropEvent = function(ev) {
   function safe(f) {
     try {
       return f();
     } catch(err) {
       return "exception: " + err.message;
     }
   }
 
   console.log("got event: " + ev.type);
 
   if (window.domAutomationController) {
     window.domAutomationController.setAutomationId(0);
     window.domAutomationController.send({
+      client_position: safe(function() {
+        return "(" + ev.clientX + ", " + ev.clientY + ")";
+      }),
       drop_effect: safe(function() { return ev.dataTransfer.dropEffect; }),
       effect_allowed: safe(function() {
         return ev.dataTransfer.effectAllowed;
       }),
       event_type: ev.type,
       file_names: safe(function() {
         return Array
             .from(ev.dataTransfer.files)
             .map(function(file) { return file.name; })
             .sort().join();
       }),
       mime_types: safe(function() {
         return Array.from(ev.dataTransfer.types).sort().join();
       }),
+      page_position: safe(function() {
+        return "(" + ev.pageX + ", " + ev.pageY + ")";
+      }),
       window_name: window.name
     });
   }
 }