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Since pdf/pdf.cc is just the implementation for an interface into the
internals code, separate it and its header out into a separate target.
Put the majority of the code in pdf/ into an "internal" target, since
that's what it is. To make the dependencies work out, also put
pdf/pdfium/pdfium_assert_matching_enums.cc into its own target.

Bug: 1103478
Change-Id: I95da94e55f1bddf957a26cece899d27222756584
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2293457
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789713}