Make DataTransferItem.getAsString() argument mandatory and nullable

LayoutTests/editing/pasteboard/data-transfer-items-drag-drop-string-expected.txt

Index: LayoutTests/editing/pasteboard/data-transfer-items-drag-drop-string-expected.txt
diff --git a/LayoutTests/editing/pasteboard/data-transfer-items-drag-drop-string-expected.txt b/LayoutTests/editing/pasteboard/data-transfer-items-drag-drop-string-expected.txt
index e9ff61ca577416ca4f518d892121b3a0a8e71a8e..5bbd710e7b2704a6f8129a69b5a3dfefb0e158a5 100644
--- a/LayoutTests/editing/pasteboard/data-transfer-items-drag-drop-string-expected.txt
+++ b/LayoutTests/editing/pasteboard/data-transfer-items-drag-drop-string-expected.txt
@@ -1,16 +1,26 @@
 This tests the basic functionality and properties of DataTransferItems for string data with drag and drop. This test requires DRT.
-Lorem ipsum 
-Drop text here if you test this manually
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
 Dragging text in source1: Lorem ipsum
 Verifying contents of DataTransferItems...
 items.length: 1
 items[0].kind: string
 items[0].type: text/plain
-PASS: "Lorem ipsum" == "Lorem ipsum"
+PASS currentItem.getAsString() threw exception TypeError: Not enough arguments.
+PASS currentItem.getAsString(null) did not throw exception.
+PASS stringData is "Lorem ipsum"
 Dragging text in source2: http://example.com
 Verifying contents of DataTransferItems...
 items.length: 1
 items[0].kind: string
 items[0].type: text/plain
-PASS: "http://example.com" == "http://example.com"
+PASS currentItem.getAsString() threw exception TypeError: Not enough arguments.
+PASS currentItem.getAsString(null) did not throw exception.
+PASS stringData is "http://example.com"
+PASS successfullyParsed is true
 
+TEST COMPLETE
+Lorem ipsum 
+Drop text here if you test this manually