DevTools: Clean up JS code and remove --jscomp_off=es5Strict compiler switch in compile_frontend.py

R=aandrey, eustas, pfeldman

Review URL: https://codereview.chromium.org/105413004

git-svn-id: svn://svn.chromium.org/blink/trunk@163352 bbb929c8-8fbe-4397-9dbb-9b2b20218538