Use the native URL.createObjectURL method more in web/firefoxcom.js

Given that `URL.createObjectURL` is assumed to always be available in MOZCENTRAL builds, note the existing usage in the file, there's no reason to depend on the PDF.js helper function `createObjectURL` at all here.

Furthermore this patch also changes `DownloadManager.downloadData` to actually revoke the `blobUrl` after downloading has completed, which is similar to the existing code in `DownloadManager.download`.

web/firefoxcom.js

@@ -14,8 +14,8 @@
  */
 
 import "../extensions/firefox/tools/l10n.js";
-import { createObjectURL, PDFDataRangeTransport, shadow } from "pdfjs-lib";
 import { DefaultExternalServices, PDFViewerApplication } from "./app.js";
+import { PDFDataRangeTransport, shadow } from "pdfjs-lib";
 import { BasePreferences } from "./preferences.js";
 import { DEFAULT_SCALE_VALUE } from "./ui_utils.js";
 
@@ -100,14 +100,23 @@ class DownloadManager {
   }
 
   downloadData(data, filename, contentType) {
-    const blobUrl = createObjectURL(data, contentType);
+    const blobUrl = URL.createObjectURL(
+      new Blob([data], { type: contentType })
+    );
+    const onResponse = err => {
+      URL.revokeObjectURL(blobUrl);
+    };
 
-    FirefoxCom.request("download", {
-      blobUrl,
-      originalUrl: blobUrl,
-      filename,
-      isAttachment: true,
-    });
+    FirefoxCom.request(
+      "download",
+      {
+        blobUrl,
+        originalUrl: blobUrl,
+        filename,
+        isAttachment: true,
+      },
+      onResponse
+    );
   }
 
   download(blob, url, filename) {