On a new package, xfstest, I forgot to run:

cros_workon start --board gnawty xfstests

When running:
emerge-gnawty xfstests

The error message was:

Calculating dependencies... done!
[ebuild  N    *] app-benchmarks/xfstests-9999 to /build/gnawty/ USE="-cros_host -profiling"

The following keyword changes are necessary to proceed:
 (see "package.accept_keywords" in the portage(5) man page for more details)
# required by xfstests (argument)
=app-benchmarks/xfstests-9999 **

NOTE: The --autounmask-keep-masks option will prevent emerge
      from creating package.unmask or ** keyword changes.

Use --autounmask-write to write changes to config files (honoring
CONFIG_PROTECT). Carefully examine the list of proposed changes,
paying special attention to mask or keyword changes that may expose
experimental or unstable packages.


Which was not helping much.