Make DataTransferItemList::length() const.

Make DataTransferItemList::length() const.
https://bugs.webkit.org/show_bug.cgi?id=76946

Just a const-correctness change.

Reviewed by Tony Chang.

No new tests since no functionality changed.

* dom/DataTransferItemList.h:
(DataTransferItemList):
* platform/chromium/DataTransferItemListChromium.cpp:
(WebCore::DataTransferItemListChromium::length):
* platform/chromium/DataTransferItemListChromium.h:
(DataTransferItemListChromium):
* platform/qt/DataTransferItemListQt.cpp:
(WebCore::DataTransferItemListQt::length):
* platform/qt/DataTransferItemListQt.h:
(DataTransferItemListQt):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105825 268f45cc-cd09-0410-ab3c-d52691b4dbfc