Update the Adminhtml image tree to use the new Serializer Json rather than Zend_Json

Author: dmanners

app/code/Magento/Cms/Block/Adminhtml/Wysiwyg/Images/Tree.php

@@ -26,20 +26,30 @@ class Tree extends \Magento\Backend\Block\Template
      */
     protected $_cmsWysiwygImages = null;
 
+    /**
+     * @var \Magento\Framework\Serialize\Serializer\Json
+     */
+    private $serializer;
+
     /**
      * @param \Magento\Backend\Block\Template\Context $context
      * @param \Magento\Cms\Helper\Wysiwyg\Images $cmsWysiwygImages
      * @param \Magento\Framework\Registry $registry
      * @param array $data
+     * @param \Magento\Framework\Serialize\Serializer\Json|null $serializer
+     * @throws \RuntimeException
      */
     public function __construct(
         \Magento\Backend\Block\Template\Context $context,
         \Magento\Cms\Helper\Wysiwyg\Images $cmsWysiwygImages,
         \Magento\Framework\Registry $registry,
-        array $data = []
+        array $data = [],
+        \Magento\Framework\Serialize\Serializer\Json $serializer = null
     ) {
         $this->_coreRegistry = $registry;
         $this->_cmsWysiwygImages = $cmsWysiwygImages;
+        $this->serializer = $serializer ?: \Magento\Framework\App\ObjectManager::getInstance()
+            ->get(\Magento\Framework\Serialize\Serializer\Json::class);
         parent::__construct($context, $data);
     }
 
@@ -65,7 +75,7 @@ public function getTreeJson()
                 'cls' => 'folder',
             ];
         }
-        return \Zend_Json::encode($jsonArray);
+        return $this->serializer->serialize($jsonArray);
     }
 
     /**