Update libpng to v1.6.55

We haven't compiled libpng into Chromium itself since
http://crrev.com/c/7128187 landed last year. However, libpng has seen
more than a few security bugs since then, and there are still some
things in the tree that depend on it, so update our copy.

Bug: 465184794
Change-Id: I4fe8f5b8eeba906bcf6c593f2cbefb2d0745514b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7645690
Auto-Submit: Matthew Riley <mattdr@google.com>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1596373}
NOKEYCHECK=True
GitOrigin-RevId: 0c8fd85b3ffd5050bb9914d2cb614881b369d2db