Linux: fix expectations for USE_SYSTEM_ZLIB

Tested with:

- zlib-1.2.7 on 64-bit Gentoo Linux
- zlib-1.2.3 on 64-bit Gentoo Linux
- zlib-1.2.3.4 on 64-bit Ubuntu Precise

BUG=none

Review URL: https://codereview.chromium.org/14087007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@194016 0039d316-1c4b-4281-b951-d872f2087c98