Make Blob#slice() contentType argument to be non nullable.

third_party/WebKit/LayoutTests/http/tests/local/fileapi/script-tests/send-sliced-dragged-file.js

 description("Test for slicing a dragged file and sending it via XMLHttpRequest.");
 
 var tempFileContent = "1234567890";
 var tempFileName = "send-slice-dragged-file.tmp";
 var subfile;
 
 function uploadFile(file, filePath, start, length, expectedException)
 {
     var xhr = new XMLHttpRequest();
     xhr.open("POST", "http://127.0.0.1:8000/resources/post-and-verify.cgi?path=" + filePath + "&start=" + start + "&length=" + length, false);
@@ skipping 92 matching lines @@
     moveMouseToCenterOfElement(fileInput);
     eventSender.mouseUp();
 }
 
 function runTest()
 {
     debug("Test slicing and sending an empty file.");
     dragAndSliceStableFile("../resources/empty.txt", 0, 0, 10);
 
     debug("Test slicing and sending a small file.");
-    dragAndSliceStableFile("../resources/file-for-drag-to-send.txt", 10, 2, 4, null);
+    dragAndSliceStableFile("../resources/file-for-drag-to-send.txt", 10, 2, 4, "");
     dragAndSliceStableFile("../resources/file-for-drag-to-send.txt", 10, 2, 20, "type/foo");
     dragAndSliceStableFile("../resources/file-for-drag-to-send.txt", 10, 15, 20, "type/bar");
 
     // This is to test a file that exceeds the read buffer limit (2K in Mac).
     debug("Test slicing and sending a big file.");
     dragAndSliceStableFile("../resources/abe.png", 12242, 10, 40);
     dragAndSliceStableFile("../resources/abe.png", 12242, 10, 3000);
     dragAndSliceStableFile("../resources/abe.png", 12242, 3000, 15000);
 
     debug("Test slicing and sending a file that has been changed right before sending.");
     dragAndSliceUnstableFile(3, 5);
 }
 
 if (window.eventSender) {
     runTest();
     // Clean up after ourselves
     removeFileInputElement();
 } else {
     testFailed("This test is not interactive, please run using DumpRenderTree");
 }
 
 var successfullyParsed = true;