filter browser logging by level of LOG

Additional fix to the next bug:
filter browser logging by level  ([35965d9](karma-runner@35965d9)), closes [karma-runner#2228](karma-runner#2228)
otherwise console.log ignore doesn't work with 
    browserConsoleLogOptions: {
        level: 'debug',
        terminal: true
    }

lib/constants.js

@@ -15,13 +15,14 @@ exports.LOG_ERROR = 'ERROR'
 exports.LOG_WARN = 'WARN'
 exports.LOG_INFO = 'INFO'
 exports.LOG_DEBUG = 'DEBUG'
+exports.LOG_LOG = 'LOG'
 exports.LOG_PRIORITIES = [
   exports.LOG_DISABLE,
   exports.LOG_ERROR,
   exports.LOG_WARN,
   exports.LOG_INFO,
   exports.LOG_DEBUG,
-  exports.LOG_DISABLE
+  exports.LOG_LOG
 ]
 
 // Default patterns for the pattern layout.