[api-minor] Replace PDFDocumentProxy.getStats with a synchronous PDFDocumentProxy.stats getter

*Please note:* These changes will primarily benefit longer documents, somewhat at the expense of e.g. one-page documents.

The existing `PDFDocumentProxy.getStats` function, which in the default viewer is called for each rendered page, requires a round-trip to the worker-thread in order to obtain the current document stats. In the default viewer, we currently make one such API-call for *every rendered* page.
This patch proposes replacing that method with a *synchronous* `PDFDocumentProxy.stats` getter instead, combined with re-factoring the worker-thread code by adding a `DocStats`-class to track Stream/Font-types and *only send* them to the main-thread *the first time* that a type is encountered.

Note that in practice most PDF documents only use a fairly limited number of Stream/Font-types, which means that in longer documents most of the `PDFDocumentProxy.getStats`-calls will return the same data.[1]
This re-factoring will obviously benefit longer document the most[2], and could actually be seen as a regression for one-page documents, since in practice there'll usually be a couple of "DocStats" messages sent during the parsing of the first page. However, if the user zooms/rotates the document (which causes re-rendering), note that even a one-page document would start to benefit from these changes.

Another benefit of having the data available/cached in the API is that unless the document stats change during parsing, repeated `PDFDocumentProxy.stats`-calls will return *the same identical* object.
This is something that we can easily take advantage of in the default viewer, by now *only* reporting "documentStats" telemetry[3] when the data actually have changed rather than once per rendered page (again beneficial in longer documents).

---
[1] Furthermore, the maximium number of `StreamType`/`FontType` are `10` respectively `12`, which means that regardless of the complexity and page count in a PDF document there'll never be more than twenty-two "DocStats" messages sent; see 41ac3f0c07/src/shared/util.js (L206-L232)

[2] One example is the `pdf.pdf` document in the test-suite, where rendering all of its 1310 pages only result in a total of seven "DocStats" messages being sent from the worker-thread.

[3] Reporting telemetry, in Firefox, includes using `JSON.stringify` on the data and then sending an event to the `PdfStreamConverter.jsm`-code.
In that code the event is handled and `JSON.parse` is used to retrieve the data, and in the "documentStats"-case we'll then iterate through the data to avoid double-reporting telemetry; see https://searchfox.org/mozilla-central/rev/8f4c180b87e52f3345ef8a3432d6e54bd1eb18dc/toolkit/components/pdfjs/content/PdfStreamConverter.jsm#515-549

web/app.js

@@ -257,6 +257,7 @@ const PDFViewerApplication = {
   _contentDispositionFilename: null,
   _contentLength: null,
   _saveInProgress: false,
+  _docStats: null,
   _wheelUnusedTicks: 0,
   _idleCallbacks: new Set(),
 
@@ -854,6 +855,7 @@ const PDFViewerApplication = {
     this._contentDispositionFilename = null;
     this._contentLength = null;
     this._saveInProgress = false;
+    this._docStats = null;
 
     this._cancelIdleCallbacks();
     promises.push(this.pdfScriptingManager.destroyPromise);
@@ -2108,6 +2110,21 @@ const PDFViewerApplication = {
     this._unblockDocumentLoadEvent = () => {};
   },
 
+  /**
+   * @ignore
+   */
+  _reportDocumentStatsTelemetry() {
+    const { stats } = this.pdfDocument;
+    if (stats !== this._docStats) {
+      this._docStats = stats;
+
+      this.externalServices.reportTelemetry({
+        type: "documentStats",
+        stats,
+      });
+    }
+  },
+
   /**
    * Used together with the integration-tests, to enable awaiting full
    * initialization of the scripting/sandbox.
@@ -2347,12 +2364,7 @@ function webViewerPageRendered({ pageNumber, error }) {
   }
 
   // It is a good time to report stream and font types.
-  PDFViewerApplication.pdfDocument.getStats().then(function (stats) {
-    PDFViewerApplication.externalServices.reportTelemetry({
-      type: "documentStats",
-      stats,
-    });
-  });
+  PDFViewerApplication._reportDocumentStatsTelemetry();
 }
 
 function webViewerPageMode({ mode }) {