Simplify sharing third_party/zlib with AOSP

Bug: https://issuetracker.google.com/115695768

Android would like to use Chromium's copy of zlib, to take advantage of
its SIMD optimizations etc. However, it does not want the renames from
chromeconf.h. Only include that file when building CHROMIUM/CHROME, so
that Android's repo can have minimal differences from this one and
build properly.

See https://android-review.googlesource.com/c/platform/external/zlib/+/979128

Change-Id: I44985166edb16ccc3333f008e0840cdd8dbce055
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648737


Reviewed-by: Chris Blume <cblume@chromium.org>
Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Commit-Queue: Adenilson Cavalcanti <cavalcantii@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#668140}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6f0e62538327e85d29118df5f546166a33c68001