PDF.js commit e293c12afcf3a9a55669b6bf095d657e7780ba16 removed the
`getPageTextContent` method from `PDFViewer`, so we need to call the
underlying method on the `PDFPageProxy` object instead.

This change is backwards compatible with earlier versions of PDF.js.

Fixes https://github.com/hypothesis/product-backlog/issues/915