[UI][STREAMING] Modify the title, 'Records' instead of 'Input Size'

## What changes were proposed in this pull request?
Spark Streaming is processing data should be record, so the title should be 'Records', and should not be 'Input Size'.

Fix before:
![1](https://user-images.githubusercontent.com/26266482/30099599-c64d4a8a-9318-11e7-8a8d-1ca99b409323.png)

Fix after:
![2](https://user-images.githubusercontent.com/26266482/30099609-cd4df7d0-9318-11e7-8a27-dbaec6797bb1.png)

## How was this patch tested?
manual tests

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

Author: guoxiaolong <[email protected]>

Closes apache#19144 from guoxiaolongzte/streamingUI.

Author: guoxiaolong

streaming/src/main/scala/org/apache/spark/streaming/ui/AllBatchesTable.scala

@@ -25,7 +25,7 @@ private[ui] abstract class BatchTableBase(tableId: String, batchInterval: Long)
 
   protected def columns: Seq[Node] = {
     <th>Batch Time</th>
-      <th>Input Size</th>
+      <th>Records</th>
       <th>Scheduling Delay
         {SparkUIUtils.tooltip("Time taken by Streaming scheduler to submit jobs of a batch", "top")}
       </th>

streaming/src/test/scala/org/apache/spark/streaming/UISeleniumSuite.scala

@@ -121,11 +121,11 @@ class UISeleniumSuite
         h4Text.exists(_.matches("Completed Batches \\(last \\d+ out of \\d+\\)")) should be (true)
 
         findAll(cssSelector("""#active-batches-table th""")).map(_.text).toSeq should be {
-          List("Batch Time", "Input Size", "Scheduling Delay (?)", "Processing Time (?)",
+          List("Batch Time", "Records", "Scheduling Delay (?)", "Processing Time (?)",
             "Output Ops: Succeeded/Total", "Status")
         }
         findAll(cssSelector("""#completed-batches-table th""")).map(_.text).toSeq should be {
-          List("Batch Time", "Input Size", "Scheduling Delay (?)", "Processing Time (?)",
+          List("Batch Time", "Records", "Scheduling Delay (?)", "Processing Time (?)",
             "Total Delay (?)", "Output Ops: Succeeded/Total")
         }