Add support for DataTransferItemList.remove() method

LayoutTests/fast/events/clipboard-dataTransferItemList-remove-expected.txt

+Checks that DataTransferItemList.remove() is working
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+* copy event
+PASS dataTansferItemList.length is 4
+PASS dataTansferItemList[0].type is "text/plain"
+PASS dataTansferItemList[1].type is "text/uri-list"
+PASS dataTansferItemList[2].type is "text/html"
+PASS dataTansferItemList[3].type is "custom-data"
+PASS dataTansferItemList.remove() threw exception TypeError: Not enough arguments.
+PASS dataTansferItemList.remove(-1) did not throw exception.
+PASS dataTansferItemList.length is 4
+PASS dataTansferItemList.remove(999) did not throw exception.
+PASS dataTansferItemList.length is 4
+PASS dataTansferItemList.remove(0) did not throw exception.
+PASS dataTansferItemList.length is 3
+PASS dataTansferItemList[0].type is "text/uri-list"
+PASS dataTansferItemList[1].type is "text/html"
+PASS dataTansferItemList.remove(-4294967295) did not throw exception.
+PASS dataTansferItemList.length is 2
+PASS dataTansferItemList[1].type is "custom-data"
+* paste event
+PASS dataTansferItemList.length is 2
+PASS dataTansferItemList.remove(0) threw exception InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable..
+PASS dataTansferItemList.length is 2
+PASS successfullyParsed is true
+
+TEST COMPLETE
+