mention MPL tri-licensed code in our copy of zlib

Review URL: https://codereview.chromium.org/1279843003

Cr-Original-Commit-Position: refs/heads/master@{#342346}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7ea99fda67bf56e494f372c6b3a278f00bed364f
1 file changed