[api-major] Only create a StatTimer for pages when enableStats == true (issue 5215)

Unless the debugging tools (i.e. `PDFBug`) are enabled, or the `browsertest` is running, the `PDFPageProxy.stats` aren't actually used for anything.
Rather than initializing unnecessary `StatTimer` instances, we can simply re-use *one* dummy class (with static methods) for every page. Note that by using a dummy `StatTimer` in this way, rather than letting `PDFPageProxy.stats` be undefined, we don't need to guard *every* single stats collection callsite.

Since it wouldn't make much sense to attempt to use `PDFPageProxy.stats` when stat collection is disabled, it was instead changed to a "private" property (i.e. `PDFPageProxy._stats`) and a getter was added for accessing `PDFPageProxy.stats`. This getter will now return `null` when stat collection is disabled, making that case easy to handle.

For benchmarking purposes, the test-suite used to re-create the `StatTimer` after loading/rendering each page. However, modifying properties on various API code from the outside in this way seems very error-prone, and is an anti-pattern that we really should avoid at all cost. Hence the `PDFPageProxy.cleanup` method was modified to accept an optional parameter, which will take care of resetting `this.stats` when necessary, and `test/driver.js` was updated accordingly.

Finally, a tiny bit more validation was added on the viewer side, to ensure that all the code we're attempting to access is defined when handling `PDFPageProxy` stats.

src/display/dom_utils.js

@@ -463,9 +463,13 @@ function isExternalLinkTargetSet() {
 
 class StatTimer {
   constructor(enable = true) {
+    this.enabled = !!enable;
+    this.reset();
+  }
+
+  reset() {
     this.started = Object.create(null);
     this.times = [];
-    this.enabled = !!enable;
   }
 
   time(name) {
@@ -513,6 +517,30 @@ class StatTimer {
   }
 }
 
+/**
+ * Helps avoid having to initialize {StatTimer} instances, e.g. one for every
+ * page, in cases where the collected stats are not actually being used.
+ * This (dummy) class can thus, since all its methods are `static`, be directly
+ * shared between multiple call-sites without the need to be initialized first.
+ *
+ * NOTE: This must implement the same interface as {StatTimer}.
+ */
+class DummyStatTimer {
+  constructor() {
+    throw new Error('Cannot initialize DummyStatTimer.');
+  }
+
+  static reset() {}
+
+  static time(name) {}
+
+  static timeEnd(name) {}
+
+  static toString() {
+    return '';
+  }
+}
+
 export {
   CustomStyle,
   RenderingCancelledException,
@@ -527,4 +555,5 @@ export {
   DOMSVGFactory,
   SimpleXMLParser,
   StatTimer,
+  DummyStatTimer,
 };