[SPARK-9777] [SQL] Window operator can accept UnsafeRows

https://issues.apache.org/jira/browse/SPARK-9777

Author: Yin Huai <yhuai@databricks.com>

Closes #8064 from yhuai/windowUnsafe and squashes the following commits:

8fb3537 [Yin Huai] Set canProcessUnsafeRows to true.

sql/core/src/main/scala/org/apache/spark/sql/execution/Window.scala

@@ -101,6 +101,8 @@ case class Window(
 
   override def outputOrdering: Seq[SortOrder] = child.outputOrdering
 
+  override def canProcessUnsafeRows: Boolean = true
+
   /**
    * Create a bound ordering object for a given frame type and offset. A bound ordering object is
    * used to determine which input row lies within the frame boundaries of an output row.