DEPS: roll zlib: 20 new commits
DEPS: roll zlib: 20 new commits

List of changes at
https://chromium.googlesource.com/chromium/src/third_party/zlib/+log/4ba7cdd0e7bf49d671645264f839838fc56e1492..c4e33043fb071b6ea0a153845da625d7ed633d3d

Of particular interest is
"Fix VC++ 2015 64-bit truncation warning in zlib".

Review URL: https://codereview.chromium.org/1416043007

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