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

third_party/WebKit/Source/core/fileapi/Blob.idl

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 24 matching lines @@
     Constructor(optional sequence<BlobPart> blobParts, optional BlobPropertyBag options),
     ConstructorCallWith=ExecutionContext,
     RaisesException=Constructor,
     Exposed=(Window,Worker),
 ] interface Blob {
     readonly attribute unsigned long long size;
     readonly attribute DOMString type;
     [RuntimeEnabled=FileAPIBlobClose] readonly attribute boolean isClosed;
 
     // TODO(jsbell): start and end arguments should be [Clamp]
-    // TODO(foolip): contentType should not be nullable.
-    [RaisesException] Blob slice(optional long long start, optional long long end, optional DOMString? contentType);
+    [RaisesException] Blob slice(optional long long start, optional long long end, optional DOMString contentType);
     [RaisesException, CallWith=ExecutionContext, RuntimeEnabled=FileAPIBlobClose] void close();
 };