[polymer] Stops using a Polymer attribute for tabLabel
[polymer] Stops using a Polymer attribute for tabLabel

There are still lots of places in the code that rely on it being an
ES6 getter/setter, so we should leave it that way for now.

BUG=catapult:#2285
TBR=aiolos@chromium.org
NOPRESUBMIT=true
NOTRY=true

Review-Url: https://codereview.chromium.org/1975313002

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