[style guide] Mark ES6 generators as approved for use
[style guide] Mark ES6 generators as approved for use

We've been using them with no problems in event_set.html for a while
now.

TBR=petrcermak@chromium.org

Review-Url: https://codereview.chromium.org/2054443003

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