Make ifdefs consistent in zib.

Debug version of Chrome is not buildable because of inconsistent #ifdefs on
NDEBUG/ASSERT/IS_DCHECK_ON . This change makes them consistemt in zlib.

BUG=666660

Review-Url: https://codereview.chromium.org/2512933002
Cr-Original-Commit-Position: refs/heads/master@{#433729}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 90a667a18ad56d14f7068e8e6cbfb5668d3b55f3