Ensure that getStructTree won't break with disableAutoFetch = true set (PR 13171 follow-up)

Open http://localhost:8888/web/viewer.html?file=/test/pdfs/pdf.pdf#disableStream=true&disableAutoFetch=true and observe the following message in the console (repeated for each page of the document):
```
Uncaught (in promise)
Object { message: "Missing data [19787293, 19787294)", name: "UnknownErrorException", details: "MissingDataException: Missing data [19787293, 19787294)", stack: "BaseExceptionClosure@http://localhost:8888/src/shared/util.js:458:29\n@http://localhost:8888/src/shared/util.js:462:3\n" }
```

src/core/document.js

@@ -454,6 +454,13 @@ class Page {
     const structTreeRoot = await this.pdfManager.ensureCatalog(
       "structTreeRoot"
     );
+    return this.pdfManager.ensure(this, "_parseStructTree", [structTreeRoot]);
+  }
+
+  /**
+   * @private
+   */
+  _parseStructTree(structTreeRoot) {
     const tree = new StructTreePage(structTreeRoot, this.pageDict);
     tree.parse();
     return tree;