Roll Zlib from 956936c53cb6 to 646b7f569718 (3 revisions)

https://chromium.googlesource.com/chromium/src/third_party/zlib.git/+log/956936c53cb6..646b7f569718

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/zlib-pdfium
Please CC pdfium-reviews+autoroller@googlegroups.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: pdfium-reviews+autoroller@googlegroups.com
Change-Id: Ic76935ae7e36a0e443ce740dc7d93161e438452f
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/116510
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 e87cef8..ba2b452 100644
--- a/DEPS
+++ b/DEPS
@@ -214,7 +214,7 @@
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling zlib
   # and whatever else without interference from each other.
-  'zlib_revision': '956936c53cb6169f2b97e009bfaefd66da227ab6',
+  'zlib_revision': '646b7f569718921d7d4b5b8e22572ff6c76f2596',
 }
 
 # Only these hosts are allowed for dependencies in this DEPS file.