I'm not sure how the pinning/unpinning works

This isn't my problem to fix.  Problems like this should go to
the Chrome OS Infra deputy:
    http://go/cros-infra-deputy

Given that this is R49, probably, Chrome hasn't been pinned.
More likely, there's a bug at work.

Same issue is failing M49 builders
21:12:04: ERROR: 
return code: 1; command: /b/cbuild/shared_internal/chromite/bin/cros_sdk 'PARALLEL_EMERGE_STATUS_FILE=/tmp/tmpZVyJT0' -- /mnt/host/source/chromite/bin/cros_extract_deps --board lulu '--format=deps' virtual/target-os virtual/target-os-dev virtual/target-os-test chromeos-base/chromeos-installshim chromeos-base/chromeos-factory chromeos-base/chromeos-hwid chromeos-base/autotest-factory-install chromeos-base/autotest-all --output-path /tmp/tmppFmqis

emerge: there are no ebuilds to satisfy ">=app-misc/c_rehash-1.7-r1" for /build/lulu/.
...

(dependency required by "chromeos-base/autotest-all" [argument])
ERROR   : emerge detected broken ebuilds. See error message above.
21:12:26: ERROR: 

e.g. https://uberchromegw.corp.google.com/i/chromeos_release/builders/auron-b-release-group%20release-R49-7834.B/builds/46


same errors seen in the PFQ
https://uberchromegw.corp.google.com/i/chromeos_release/builders/lumpy-pre-flight-branch%20release-R49-7834.B/builds/146/steps/BuildPackages/logs/stdio


i don't see a recent change that may have caused this

looking in more details it seems like this is caused by https://chromium-review.googlesource.com/#/c/324990/

reverted it and re running pfq/builds 

ok, things are working now after above CL was reverted 

Mike, we can follow up in   Issue 567688  

Bulk verified