Process null capabilities as a capability fetch failure.

chrome/browser/resources/print_preview/native_layer.js

 // Copyright (c) 2012 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.
 
 cr.exportPath('print_preview');
 
 /**
  * @typedef {{selectSaveAsPdfDestination: boolean,
  *            layoutSettings.portrait: boolean,
  *            pageRange: string,
@@ skipping 532 matching lines @@
       this.dispatchEvent(localDestsSetEvent);
     },
 
     /**
      * Called when native layer gets settings information for a requested local
      * destination.
      * @param {Object} settingsInfo printer setting information.
      * @private
      */
     onUpdateWithPrinterCapabilities_: function(settingsInfo) {
+      if (settingsInfo.capabilities == null) {

[dpapad, 2017/05/01 17:39:28]

Why are we doing this in Javascript instead of C++? At [1] there is already code
that checks settingsInfo and calls the error callback directly. Here we let C++
call the success callback, and redirect to the error callback ourselves. It
seems unnecessary.

[1]
https://cs.chromium.org/chromium/src/chrome/browser/ui/webui/print_preview/pr...

[skau, 2017/05/01 22:51:26]

It crept up as a bug.  The code you referenced only checks that settingsInfo is
non-null.  But in fact, we get crashes if settingsInfo.capabilities is null. 
However, I'll make this an assert and strengthen the check in the handler. 

+        this.onFailedToGetPrinterCapabilities_(settingsInfo['printerId']);
+        return;
+      }
+
       var capsSetEvent = new Event(NativeLayer.EventType.CAPABILITIES_SET);
       capsSetEvent.settingsInfo = settingsInfo;
       this.dispatchEvent(capsSetEvent);
     },
 
     /**
      * Called when native layer gets settings information for a requested local
      * destination.
      * @param {string} destinationId Printer affected by error.
      * @private
@@ skipping 541 matching lines @@
       return this.serializedDefaultDestinationSelectionRulesStr_;
     }
   };
 
   // Export
   return {
     NativeInitialSettings: NativeInitialSettings,
     NativeLayer: NativeLayer
   };
 });