[vinn] Change read() to readbuffer() in d8_bootstrap's file exist polyfill
[vinn] Change read() to readbuffer() in d8_bootstrap's file exist polyfill

This improve the runtime of vinn in general since readbuffer is faster than read.

TBR=nduca@chromium.org

Review URL: https://codereview.chromium.org/1751993002

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