[Cleanup] Removing dead code in multiprocessing_bootstrap.py

Removing gtk related code in multiprocessing_bootstrap.py

Review URL: https://codereview.chromium.org/483143003

git-svn-id: svn://svn.chromium.org/blink/trunk@180599 bbb929c8-8fbe-4397-9dbb-9b2b20218538