Add simple script for unlocking keychain on Macs.
Add simple script for unlocking keychain on Macs.

BUG=chromium:632392

Review-Url: https://codereview.chromium.org/2190113002

catapult-project/catapult-csm,benschmaus/catapult,sahiljain/catapult,sahiljain/catapult,catapult-project/catapult,sahiljain/catapult,catapult-project/catapult,sahiljain/catapult,catapult-project/catapult-csm,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,benschmaus/catapult,sahiljain/catapult,catapult-project/catapult-csm,catapult-project/catapult,sahiljain/catapult,benschmaus/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult-csm,benschmaus/catapult,catapult-project/catapult-csm,benschmaus/catapult,benschmaus/catapult