DevTools: Parallelize module-wise compilation in compile_frontend.py

If more than 8G of RAM are available, the modules will be compiled in parallel,
with the nice level of 10 (since the number of modules can exceed the number of cores).
Parallel compilation can be forced by setting the environment variable
FORCE_PARALLEL_DEVTOOLS_COMPILATION=1.

This gives a rougly 50% performance gain (47s vs 101s on the local machine).

R=aandrey, eustas, vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/103893008

git-svn-id: svn://svn.chromium.org/blink/trunk@164155 bbb929c8-8fbe-4397-9dbb-9b2b20218538