Roll libpng from dc2507e99733 to 84d676669166 (4 revisions)

https://chromium.googlesource.com/chromium/src/third_party/libpng.git/+log/dc2507e99733..84d676669166

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/libpng-pdfium
Please CC thestig@chromium.org,thestig@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://issues.skia.org/issues/new?component=1389291&template=1850622

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

Bug: None
Tbr: thestig@google.com
Change-Id: I921a7ef75239fe2ed55dda1b30909e5adfe5daf1
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/137350
Bot-Commit: pdfium-autoroll <pdfium-autoroll@skia-public.iam.gserviceaccount.com>
Commit-Queue: pdfium-autoroll <pdfium-autoroll@skia-public.iam.gserviceaccount.com>
diff --git a/DEPS b/DEPS
index 3d8e321..95c0654 100644
--- a/DEPS
+++ b/DEPS
@@ -168,7 +168,7 @@
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling libpng
   # and whatever else without interference from each other.
-  'libpng_revision': 'dc2507e99733e9e3bfa7b4387a3755fd8ee77542',
+  'libpng_revision': '84d676669166912cb76f8636526c77c2ac1cd069',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling libunwind
   # and whatever else without interference from each other.