DevTools: Improve Closure compiler running times in compile_frontend.py

Use option 2 recommended at https://code.google.com/p/closure-compiler/wiki/FAQ#What_are_the_recommended_Java_VM_command-line_options?
This shaves about 30% off the compile_frontend.py execution time (~9.5s vs ~14.5s).

R=aandrey, eustas, vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/112883002

git-svn-id: svn://svn.chromium.org/blink/trunk@163775 bbb929c8-8fbe-4397-9dbb-9b2b20218538