Same failure appeared on 10 of the builders but not the others.  One example partial log shown below.

Link to build or pfq page.
https://uberchromegw.corp.google.com/i/chromeos/builders/betty-chrome-pfq/builds/1299

Snippet of log that contains the failure.
02:12:39: INFO: RunCommand: /b/c/cbuild/repository/chromite/bin/cros_sdk -- emerge-betty -p --cols --quiet --root /var/empty -e virtual/target-os in /b/c/cbuild/repository
02:13:07: ERROR: Traceback (most recent call last):
  File "/b/c/cbuild/repository/chromite/cbuildbot/stages/generic_stages.py", line 701, in Run
    self.PerformStage()
  File "/b/c/cbuild/repository/chromite/cbuildbot/stages/android_stages.py", line 158, in PerformStage
    versions, branches = self._UpdateBoardDictsForAndroidBuildInfo()
  File "/b/c/cbuild/repository/chromite/cbuildbot/stages/android_stages.py", line 133, in _UpdateBoardDictsForAndroidBuildInfo
    version = self._run.DetermineAndroidVersion(boards=[board])
  File "/b/c/cbuild/repository/chromite/cbuildbot/cbuildbot_run.py", line 978, in FuncWrapper
    return result(*args, **kwargs)
  File "/b/c/cbuild/repository/chromite/cbuildbot/cbuildbot_run.py", line 887, in DetermineAndroidVersion
    package = self.DetermineAndroidPackage(board)
  File "/b/c/cbuild/repository/chromite/cbuildbot/cbuildbot_run.py", line 860, in DetermineAndroidPackage
    packages = portage_util.GetPackageDependencies(board, 'virtual/target-os')
  File "/b/c/cbuild/repository/chromite/lib/portage_util.py", line 1846, in GetPackageDependencies
    capture_output=True).output.splitlines()
  File "/b/c/cbuild/repository/chromite/lib/cros_build_lib.py", line 658, in RunCommand
    raise RunCommandError(msg, cmd_result)
RunCommandError: return code: 1; command: /b/c/cbuild/repository/chromite/bin/cros_sdk -- emerge-betty -p --cols --quiet --root /var/empty -e virtual/target-os
The following keyword changes are necessary to proceed:
 (see "package.accept_keywords" in the portage(5) man page for more details)
# required by chromeos-base/chromeos-cheets-0.0.1-r25::cheets-private
# required by virtual/cheets-1.7-r1::cheets-private
# required by virtual/target-chromium-os-1-r88::chromiumos
# required by virtual/target-chrome-os-1-r17::chromeos
# required by virtual/target-os-1.5-r1::chromeos
# required by virtual/target-os (argument)

stevenjb@ - Could you take a look and advise?  You're the owner of  Issue 730999  and Issue 729608 (although I don't think this second one is relevant), both of which show this same error.