DevTools: Parallelize compile_frontend.py

This change parallelizes compilation phases for the general frontend code,
injected script, and generated canvas module code.

compile_frontend.py performance:
  Without patch: avg=20.458s, stdev=0.835s
  With patch applied: avg=15.174s, stdev=0.397s

Drive-by: do not dump the java executable path into stdout

R=aandrey, eustas, vsevik, yurys
NOTRY=true

Review URL: https://codereview.chromium.org/109173003

git-svn-id: svn://svn.chromium.org/blink/trunk@163534 bbb929c8-8fbe-4397-9dbb-9b2b20218538