[SPARK-23867][SCHEDULER] use droppedCount in logWarning

## What changes were proposed in this pull request?

Get the count of dropped events for output in log message.

## How was this patch tested?

The fix is pretty trivial, but `./dev/run-tests` were run and were successful.

Please review http://spark.apache.org/contributing.html before opening a pull request.

vanzin cloud-fan

The contribution is my original work and I license the work to the project under the project’s open source license.

Author: Patrick Pisciuneri <Patrick.Pisciuneri@target.com>

Closes #20977 from phpisciuneri/fix-log-warning.

core/src/main/scala/org/apache/spark/scheduler/AsyncEventQueue.scala

@@ -166,7 +166,7 @@ private class AsyncEventQueue(val name: String, conf: SparkConf, metrics: LiveLi
           val prevLastReportTimestamp = lastReportTimestamp
           lastReportTimestamp = System.currentTimeMillis()
           val previous = new java.util.Date(prevLastReportTimestamp)
-          logWarning(s"Dropped $droppedEvents events from $name since $previous.")
+          logWarning(s"Dropped $droppedCount events from $name since $previous.")
         }
       }
     }