Improve libpng visibility list for PDFium

Change the visibility list to account for both standalone PDFium, and
PDFium in Chromium. Separate this out into its own list, so readers can
more easily see it is the same dependency, but from two different
checkouts. Also remove the wildcard and more narrowly allow dependencies
only from the "png" target.

Bug: 433723342
Change-Id: I81b9153dacc0c36de321c64df36a6162a8af2131
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6819898
Reviewed-by: Chris Blume <cblume@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1496989}