Some minor cleanup after SPARK-4550.

JoshRosen this PR addresses the comments you left on apache#4450 after it got merged.

Author: Sandy Ryza <sandy@cloudera.com>

Closes apache#5916 from sryza/sandy-spark-4550-cleanup and squashes the following commits:

dee3d85 [Sandy Ryza] Some minor cleanup after SPARK-4550.

core/src/main/scala/org/apache/spark/storage/BlockObjectWriter.scala

@@ -59,7 +59,7 @@ private[spark] abstract class BlockObjectWriter(val blockId: BlockId) extends Ou
   def write(key: Any, value: Any)
 
   /**
-   * Notify the writer that a record worth of bytes has been written with writeBytes.
+   * Notify the writer that a record worth of bytes has been written with OutputStream#write.
    */
   def recordWritten()
 
@@ -215,12 +215,7 @@ private[spark] class DiskBlockObjectWriter(
 
     objOut.writeKey(key)
     objOut.writeValue(value)
-    numRecordsWritten += 1
-    writeMetrics.incShuffleRecordsWritten(1)
-
-    if (numRecordsWritten % 32 == 0) {
-      updateBytesWritten()
-    }
+    recordWritten()
   }
 
   override def write(b: Int): Unit = throw new UnsupportedOperationException()

core/src/main/scala/org/apache/spark/util/collection/PartitionedSerializedPairBuffer.scala

@@ -71,10 +71,10 @@ private[spark] class PartitionedSerializedPairBuffer[K, V](
     if (keyStart < 0) {
       throw new Exception(s"Can't grow buffer beyond ${1 << 31} bytes")
     }
-    kvSerializationStream.writeObject[Any](key)
+    kvSerializationStream.writeKey[Any](key)
     kvSerializationStream.flush()
     val valueStart = kvBuffer.size
-    kvSerializationStream.writeObject[Any](value)
+    kvSerializationStream.writeValue[Any](value)
     kvSerializationStream.flush()
     val valueEnd = kvBuffer.size