WebView enters a debug mode, if java.util.logging is used with the attached logging.properties (System.setProperty("java.util.logging.config.file", "config/logging.properties");).
This can be reproduced with the JavaFX 2 Ensemble WebView example.

Workaround posted by Brad Johnson:

Logger logger =
            Logger.getLogger( WCGraphicsPrismContext.class.getName() );
logger.setLevel( Level.OFF );