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Don't allow changing GSSAPI library name on Chrome OS (there's no
reason to). Note that the GSSAPILibraryName policy is not supported
on Chrome OS, which means that it already cannot be set through
policy. This CL is an additional security measure.

BUG=chromium:755153
TEST=Compiled, trybots

Change-Id: Ibc2eb7e0ce59ccc5e7f7c8fd66078bc0efb2e3a5
Reviewed-on: https://chromium-review.googlesource.com/685836Reviewed-by: Nicolas Zea <zea@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Lutz Justen <ljusten@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505297}