Invoke idl_compiler.py with -S (improves build time 10%)

Python startup time is typically 15 ms (real), mostly due to the default
import site.
This can be skipped by using the -S option, which cuts startup time to
4 ms, so about 6.5 seconds total, over 10% of time on my box.

BUG=341748
TBR=haraken

Review URL: https://codereview.chromium.org/190273003

git-svn-id: svn://svn.chromium.org/blink/trunk@168724 bbb929c8-8fbe-4397-9dbb-9b2b20218538