Implement icon attribute for menuitem

LayoutTests/http/tests/misc/custom-context-menu-expected.txt

+Tests the custom context menu items.
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+PASS itemsWithCustomContextMenu.length - items.length is 24
+PASS itemsWithCustomContextMenu[0] is "Item1"
+PASS itemsWithCustomContextMenu[1] is "#Item2"
+PASS itemsWithCustomContextMenu[2] is "---------"
+PASS itemsWithCustomContextMenu[3] is "Item3"
+PASS itemsWithCustomContextMenu[4] is "Submenu >"
+PASS itemsWithCustomContextMenu[5] is "_Item4"
+PASS itemsWithCustomContextMenu[6] is "_Deeper submenu >"
+PASS itemsWithCustomContextMenu[7] is "__Item5"
+PASS itemsWithCustomContextMenu[8] is "__---------"
+PASS itemsWithCustomContextMenu[9] is "__Item6"
+PASS itemsWithCustomContextMenu[10] is "Item7"
+PASS itemsWithCustomContextMenu[11] is "---------"
+PASS itemsWithCustomContextMenu[12] is "Item8"
+PASS itemsWithCustomContextMenu[13] is "Item11"
+PASS itemsWithCustomContextMenu[14] is "Item12"
+PASS itemsWithCustomContextMenu[15] is "---------"
+PASS itemsWithCustomContextMenu[16] is "Item13"
+PASS itemsWithCustomContextMenu[17] is "Item14"
+PASS itemsWithCustomContextMenu[18] is "*Item15"
+PASS itemsWithCustomContextMenu[19] is "Item16"
+PASS itemsWithCustomContextMenu[20] is "Item17"
+PASS itemsWithCustomContextMenu[21] is "*Item18"
+PASS itemsWithCustomContextMenu[22] is "Item19"
+PASS itemsWithCustomContextMenu[23] is "Item20http://127.0.0.1:8000/misc/abc.png"
+PASS items1.length - items.length is 0
+PASS items2.length - items.length is 0
+PASS items3.length - items.length is 0
+PASS items4.length - items.length is 0
+PASS items5.length - items.length is 3
+PASS items5[0] is "Item1"
+PASS items5[1] is "Item2"
+PASS items5[2] is "Item3"
+PASS successfullyParsed is true
+
+TEST COMPLETE
+   
+      
+