DevTools: Run closure compiler for all target modules in multiple threads

This patch enables compile_frontend.py to run the closure compiler for all required modules
in multiple threads rather than in multiple processes (which consume additional memory
for the heap and additional time for the JVM startup).

Performance:
compile_frontend.py all:
  - before: 53s
  - after (the only option now): 13s

Drive-by: Fix nullability check for several @type casts in a single line.

R=pfeldman@chromium.org, pfeldman, sergeyv

Review URL: https://codereview.chromium.org/145033002

git-svn-id: svn://svn.chromium.org/blink/trunk@165643 bbb929c8-8fbe-4397-9dbb-9b2b20218538