Copy over image sanitizer and dimension extractor to avoid fatal errors

includes/AMP/Image_Dimension_Extractor.php

@@ -0,0 +1,51 @@
+<?php
+/**
+ * Class Image_Dimension_Extractor.
+ *
+ * @package   Google\Web_Stories
+ * @copyright 2020 Google LLC
+ * @license   https://www.apache.org/licenses/LICENSE-2.0 Apache License 2.0
+ * @link      https://github.com/google/web-stories-wp
+ */
+
+/**
+ * Copyright 2020 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace Google\Web_Stories\AMP;
+
+use AMP_Image_Dimension_Extractor;
+
+/**
+ * Image Dimension Extractor class.
+ *
+ * Class with static methods to extract image dimensions.
+ *
+ * This version avoids using the AMP__VERSION constant.
+ *
+ * @since 1.0.0
+ */
+class Image_Dimension_Extractor extends AMP_Image_Dimension_Extractor {
+	/**
+	 * Get default user agent.
+	 *
+	 * @since 1.0.0
+	 *
+	 * @return string
+	 */
+	public static function get_default_user_agent() {
+		return 'web-stories-wp, v' . WEBSTORIES_VERSION . ', ' . home_url();
+	}
+}