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Some packages used in linux builds are not used on CrOS, and
indeed are not even installed in the CrOS build chroot. Exclude
them so that the GN build doesn't try to run pkg-config on
packages that aren't installed.

BUG=388412
TEST=Create a target that builds //mojo/public, build with os==chromeos
R=brettw

Review URL: https://codereview.chromium.org/556733002

Cr-Commit-Position: refs/heads/master@{#293977}