playwright: add playwright bench for chrome

[krichprollsch]

Result on my machine

$ RUNS=200 CHROME_PATH=`which google-chrome` npm run bench-chrome

> demo@1.0.0 bench-chrome
> node playwright/chrome.js

................................................................................
................................................................................
........................................
total runs 200
total duration (ms) 20235
avg run duration (ms) 100
min run duration (ms) 89
max run duration (ms) 163

fix #4