[SPARK-6138][CORE][minor] enhance the `toArray` method in `SizeTrackingVector`

Use array copy instead of `Iterator#toArray` to make it more efficient.

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #4825 from cloud-fan/minor and squashes the following commits:

c933ee5 [Wenchen Fan] make toArray method just in parent
946a35b [Wenchen Fan] minor enhance

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

@@ -71,12 +71,21 @@ class PrimitiveVector[@specialized(Long, Int, Double) V: ClassTag](initialSize:
 
   /** Resizes the array, dropping elements if the total length decreases. */
   def resize(newLength: Int): PrimitiveVector[V] = {
-    val newArray = new Array[V](newLength)
-    _array.copyToArray(newArray)
-    _array = newArray
+    _array = copyArrayWithLength(newLength)
     if (newLength < _numElements) {
       _numElements = newLength
     }
     this
   }
+
+  /** Return a trimmed version of the underlying array. */
+  def toArray: Array[V] = {
+    copyArrayWithLength(size)
+  }
+
+  private def copyArrayWithLength(length: Int): Array[V] = {
+    val copy = new Array[V](length)
+    _array.copyToArray(copy)
+    copy
+  }
 }

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

@@ -36,11 +36,4 @@ private[spark] class SizeTrackingVector[T: ClassTag]
     resetSamples()
     this
   }
-
-  /**
-   * Return a trimmed version of the underlying array.
-   */
-  def toArray: Array[T] = {
-    super.iterator.toArray
-  }
 }