Roll libpng from 34954dfe60ef to 68c9960bdc0a (1 revision)

https://chromium.googlesource.com/chromium/src/third_party/libpng.git/+log/34954dfe60ef..68c9960bdc0a

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 awscreen@google.com,thestig@chromium.org 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: awscreen@google.com
Change-Id: I48b628b5244410dd3aadfa8cbeb6b02150552c88
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/123250
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 6e7c9cc..b2318d18 100644
--- a/DEPS
+++ b/DEPS
@@ -150,7 +150,7 @@
   # 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': '34954dfe60ef2cb2c805792e1f08ff7db067a861',
+  'libpng_revision': '68c9960bdc0aa723a9481ab848fd928604d928e9',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling libunwind
   # and whatever else without interference from each other.