This lets other projects reuse the plain parts of third_party/zlib
without depending on Chromium's base/.

BUG=541704

Review-Url: https://codereview.chromium.org/2781353002
Cr-Original-Commit-Position: refs/heads/master@{#460863}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: da0685cc8b20ccce91494583f91aa4a8723ad7c3