MD Settings: Display: Fix runtime error

chrome/browser/resources/settings/device_page/layout_behavior.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.
 
 /**
  * @fileoverview Behavior for handling display layout, specifically
  *     edge snapping and collisions.
  */
 
 /** @polymerBehavior */
@@ skipping 63 matching lines @@
     }
   },
 
   /**
    * Called when a drag event occurs. Checks collisions and updates the layout.
    * @param {string} id
    * @param {!chrome.system.display.Bounds} newBounds The new calculated
    *     bounds for the display.
    * @return {!chrome.system.display.Bounds}
    */
-  updateDisplayBounds(id, newBounds) {
+  updateDisplayBounds: function(id, newBounds) {
     this.dragLayoutId = id;
 
     // Find the closest parent.
     var closestId = this.findClosest_(id, newBounds);

[dpapad, 2016/11/23 18:18:43]

Nit (optional):
var closestId = assert(this.findClosest_(id, newBounds));

[stevenjb, 2016/11/23 18:44:32]

Personal nit: I'm not a huge fan of wrapping essential logic inside of an assert
outside of tests. Partly because in C++ if you accidentally do that with a
DCHECK the logic is never run in production code, but also because I find it
harder to read. We rarely do that in Chrome C++ code, although I realize this
may be a common pattern in JS.

[Dan Beam, 2016/11/23 20:46:55]

JS != C++

+    assert(closestId);
 
     // Find the closest edge.
     var closestBounds = this.getCalculatedDisplayBounds(closestId);
     var layoutPosition =
         this.getLayoutPositionForBounds_(newBounds, closestBounds);
 
     // Snap to the closest edge.
     var snapPos = this.snapBounds_(newBounds, closestId, layoutPosition);
     newBounds.left = snapPos.x;
     newBounds.top = snapPos.y;
@@ skipping 22 matching lines @@
 
     this.dragBounds_ = newBounds;
 
     return newBounds;
   },
 
   /**
    * Called when dragging ends. Sends the updated layout to chrome.
    * @param {string} id
    */
-  finishUpdateDisplayBounds(id) {
+  finishUpdateDisplayBounds: function(id) {
     this.highlightEdge_('', undefined);  // Remove any highlights.
     if (id != this.dragLayoutId || !this.dragBounds_ ||
         !this.dragLayoutPosition_) {
       return;
     }
 
     var layout = this.displayLayoutMap_.get(id);
 
     var orphanIds;
     if (!layout || layout.parentId == '') {
@@ skipping 556 matching lines @@
           highlight == chrome.system.display.LayoutPosition.LEFT);
       div.classList.toggle(
           'highlight-top',
           highlight == chrome.system.display.LayoutPosition.TOP);
       div.classList.toggle(
           'highlight-bottom',
           highlight == chrome.system.display.LayoutPosition.BOTTOM);
     }
   },
 };