Roll third_party/zlib/ 1b274918e..21c6af6f1 (7 commits)

https://chromium.googlesource.com/chromium/src/third_party/zlib.git/+log/1b274918e40f..21c6af6f18e5

Created with:
  roll-dep third_party/zlib

TBR=tsepez@chromium.org

Change-Id: Ie7645fda5314908c448638a634ae7ff406242998
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/69330
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/DEPS b/DEPS
index 8bd692e..40a8c32 100644
--- a/DEPS
+++ b/DEPS
@@ -103,7 +103,7 @@
   # 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': '1b274918e40f45b80d39173919e1ef5550dc40a6',
+  'zlib_revision': '21c6af6f18e5ad9c3b9d103f8cfdbf926ca35f7c',
 }
 
 deps = {