Remove the constructor in the `StatTimer` class
Remove the constructor in the `StatTimer` class

With modern EcmaScript features, we can define these fields directly instead. Please note that for backwards compatibility purposes they are still public as before, however note that this functionality is *disabled* by default (see the `pdfBug` API option).
Also, we can (slightly) simplify the two loops used in the `toString` method.

Shoobx/pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js,showpad/mozilla-pdf.js,Snuffleupagus/pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js,Shoobx/pdf.js,Snuffleupagus/pdf.js,Snuffleupagus/pdf.js,Shoobx/pdf.js,mozilla/pdf.js