[Streaming][Minor]Replace some 'if-else' in Clock

Replace some 'if-else' statement by math.min and math.max in Clock.scala

Author: huangzhaowei <carlmartinmax@gmail.com>

Closes apache#3088 from SaintBacchus/StreamingClock and squashes the following commits:

7b7f8e7 [huangzhaowei] [Streaming][Minor]Replace some 'if-else' in Clock

streaming/src/main/scala/org/apache/spark/streaming/util/Clock.scala

@@ -41,26 +41,15 @@ class SystemClock() extends Clock {
       return currentTime
     }
 
-    val pollTime = {
-      if (waitTime / 10.0 > minPollTime) {
-        (waitTime / 10.0).toLong
-      } else {
-        minPollTime
-      }
-    }
+    val pollTime = math.max(waitTime / 10.0, minPollTime).toLong
 
     while (true) {
       currentTime = System.currentTimeMillis()
       waitTime = targetTime - currentTime
       if (waitTime <= 0) {
         return currentTime
       }
-      val sleepTime =
-        if (waitTime < pollTime) {
-          waitTime
-        } else {
-          pollTime
-        }
+      val sleepTime = math.min(waitTime, pollTime)
       Thread.sleep(sleepTime)
     }
     -1