Working on PR 20784, I couldn't help noticing that this code can be improved a little bit.
- Only initialize `PdfTextExtractor` in development mode and MOZCENTRAL builds, since it's unused elsewhere.
- Re-factor how `PdfTextExtractor` waits for the viewer to be available/ready, by using existing (internal) events.
This simplifies the `PdfTextExtractor` class and removes its `setViewer` method, which improves general consistency since normally the viewer-components don't use such a method in that way (here it was effectively used as a stand-in for a `setDocument` method).
- Finally, while slightly unrelated, rename the `#getAllTextInProgress` field in the `PDFViewer` class to `#copyAllInProgress` to clearly indicate what it's for since the `getAllText` method is used more generally now.
