Use offsetX and offsetY to position the fake mouse cursor.

The mouse cursor element is positioned relative to the document body (ie, ignoring
the title bar for apps v2). However, its position was being set using the x and y
coordinates of the mouse event, which are relative to the client viewport. This CL
fixes this, using offsetX/Y, which are relative to the target element.

BUG=410473

Review URL: https://codereview.chromium.org/923013004

Cr-Commit-Position: refs/heads/master@{#317447}

remoting/webapp/crd/js/desktop_connected_view.js

@@ -79,8 +79,8 @@ remoting.DesktopConnectedView = function(session, container, host, mode,
   var img = this.mouseCursorOverlay_;
   /** @param {Event} event @private */
   this.updateMouseCursorPosition_ = function(event) {
-    img.style.top = event.y + 'px';
-    img.style.left = event.x + 'px';
+    img.style.top = event.offsetY + 'px';
+    img.style.left = event.offsetX + 'px';
   };
 
   /** @type {number?} @private */