Update zlib to 470d3a2ee4ef721688ce6961bc865a99fcb64070.

Change-Id: I0254f83d57607f500811a6dab9bf6f11125ccee7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/434280
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: rmacnak-google

DEPS

@@ -114,7 +114,7 @@ vars = {
   "ply_rev": "604b32590ffad5cbb82e4afef1d305512d06ae93",
   "protobuf_gn_rev": "ca669f79945418f6229e4fef89b666b2a88cbb10",
   "WebCore_rev": "bcb10901266c884e7b3740abc597ab95373ab55c",
-  "zlib_rev": "108fa50cda23ed4a712a098d058dccbbfd248206",
+  "zlib_rev": "470d3a2ee4ef721688ce6961bc865a99fcb64070",
 
   # Note: Updates to dart_style have to be coordinated with the infrastructure
   # team so that the internal formatter `tools/sdks/dart-sdk/bin/dart format`