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- For input and generated PDFs, validate all the page sizes.
- Set the page size / printable area to 72 DPI, which is what the
  PdfNupConverterClient normally receives.

Bug: 1004529
Change-Id: I615b014c08332a55c395f06352424fe1c96a4601
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1895071
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711884}