Do not skip site import on python startup.

Remove the -S option when invoking the bindings generator to allow implicit 'import site' by the python runtime.

BUG=410787

Review URL: https://codereview.chromium.org/538913003

git-svn-id: svn://svn.chromium.org/blink/trunk@181375 bbb929c8-8fbe-4397-9dbb-9b2b20218538