[api-minor] Add "contentLength" to the information returned by the getMetadata method

Given that we already include the "Content-Disposition"-header filename, when it exists, it shouldn't hurt to also include the information from the "Content-Length"-header.
For PDF documents opened via a URL, which should be a very common way for the PDF.js library to be used, this will[1] thus provide a way of getting the PDF filesize without having to wait for the `getDownloadInfo`-promise to resolve[2].

With these API improvements, we can also simplify the filesize handling in the `PDFDocumentProperties` class.

---
[1] Assuming that the server is correctly configured, of course.

[2] Since that's not *guaranteed* to happen in general, with e.g. `disableAutoFetch = true` set.

web/app.js

@@ -863,15 +863,10 @@ const PDFViewerApplication = {
       }
       parameters[key] = value;
     }
-
+    // Finally, update the API parameters with the arguments (if they exist).
     if (args) {
       for (const key in args) {
-        const value = args[key];
-
-        if (key === "length") {
-          this.pdfDocumentProperties.setFileSize(value);
-        }
-        parameters[key] = value;
+        parameters[key] = args[key];
       }
     }