Change async_slice_model to use generators instead of iterators.
Change async_slice_model to use generators instead of iterators.

Import time for android_systrace.html (9 trials each):

Before: min=0.728s, median=1.227s, max=3.128s
After: min=0.878s, median=1.390s, max=2.700s

BUG=catapult:#2407

Review-Url: https://codereview.chromium.org/2065283003

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