Fix the use of pdfium

FPDF_LoadPage returns FPDF_PAGE and that's also what expected by other
APIs. There's no need for conversion.

This works now because FPDF_PAGE is defined as void*. But will fails
with new pdfium.

Bug: 155031873
Change-Id: I74381ec8ec36797a5901ea3de845c2b5d798ba0c