Handle null coordinates from XYZ named destination from a URL.

Currently PDFiumEngine::GetNamedDestination() uses the public API
FPDFDest_GetView() to get a named destination's view, which outputs 0 if
a parameter is null. However, for the x and y coordinates from an XYZ
named destination, 0 is a valid input while null should be treated as
an empty input.

This CL adds a string `xyz_params` in PDFEngine::NamedDestination so
that null parameters can be passed to the viewport and get treated
differently from 0 for x and y coordinates.

Bug: 1157061, 1163701
Change-Id: I81a5a7c4b9c895263e580473a68a070e8b7965dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2634130
Commit-Queue: Hui Yingst <nigi@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845914}

• chrome/browser/resources/pdf/open_pdf_params_parser.js[diff]
• chrome/browser/resources/pdf/viewport.js[diff]
• chrome/test/data/pdf/params_parser_test.js[diff]
• pdf/out_of_process_instance.cc[diff]
• pdf/pdf_engine.h[diff]
• pdf/pdfium/pdfium_engine.cc[diff]