[PDF Ink Signatures] Refactor common code into pdfium_ink_transform.cc

Take duplicate code that exists in both pdfium_ink_reader.cc and
pdfium_ink_writer.cc and combine them into a helper function named
GetCanonicalToPdfTransformForPage() that lives in
pdfium_ink_transform.cc. Add unit tests to check the transforms work as
expected on some existing PDFs.

Change-Id: I0497c995be9d7a69177c646e4ec1bcd6ab9efcd1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6816387
Reviewed-by: Andy Phan <andyphan@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1497072}