[histogram-span] Format underflow bin name using unit for consistency.
[histogram-span] Format underflow bin name using unit for consistency.

Before: https://screenshot.googleplex.com/usvGPQDSLmi.png
After: https://screenshot.googleplex.com/X0PucE8amUe.png

BUG=catapult:#2928

Review-Url: https://codereview.chromium.org/2417503002

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