logging: restore call to log_message_handler::Install()

I missed this when moving shared components to SystemRewriteDriverFactory;
this call used to be in NgxRewriteDriverFactory::RootInit.  Without it we only
log VLOG(0), not VLOG(1) and VLOG(2) the way Apache does.

Fixes #518.
logging: remove unused constant
Merge pull request #536 from pagespeed/jefftk-vlogs

logging: restore call to log_message_handler::Install()