Add loading attribute for HTMLIframeElement (microsoft#1221)

Co-authored-by: saschanaz <saschanaz@users.noreply.github.com>

Author: Shogo Sensui

baselines/dom.generated.d.ts

@@ -11501,8 +11501,12 @@ interface HTMLIFrameElement extends HTMLElement {
      * [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLIFrameElement/height)
      */
     height: string;
-    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLIFrameElement/loading) */
-    loading: string;
+    /**
+     * Sets or retrieves the policy for loading iframe elements that are outside the viewport.
+     *
+     * [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLIFrameElement/loading)
+     */
+    loading: "eager" | "lazy";
     /**
      * Sets or retrieves a URI to a long description of the object.
      * @deprecated

baselines/ts5.5/dom.generated.d.ts

@@ -11488,8 +11488,12 @@ interface HTMLIFrameElement extends HTMLElement {
      * [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLIFrameElement/height)
      */
     height: string;
-    /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLIFrameElement/loading) */
-    loading: string;
+    /**
+     * Sets or retrieves the policy for loading iframe elements that are outside the viewport.
+     *
+     * [MDN Reference](https://developer.mozilla.org/docs/Web/API/HTMLIFrameElement/loading)
+     */
+    loading: "eager" | "lazy";
     /**
      * Sets or retrieves a URI to a long description of the object.
      * @deprecated

inputfiles/addedTypes.jsonc

@@ -927,6 +927,10 @@
                     "property": {
                         "referrerPolicy": {
                             "type": "ReferrerPolicy"
+                        },
+                        "loading": {
+                            "name": "loading",
+                            "overrideType": "\"eager\" | \"lazy\""
                         }
                     }
                 }

inputfiles/comments.json

@@ -1015,6 +1015,9 @@
                         "contentDocument": {
                             "comment": "Retrieves the document object of the page or frame."
                         },
+                        "loading": {
+                            "comment": "Sets or retrieves the policy for loading iframe elements that are outside the viewport."
+                        },
                         "longDesc": {
                             "comment": "Sets or retrieves a URI to a long description of the object."
                         }