Convert DataTransferItem/DataTransferItemList back into an interface class

Convert DataTransferItem/DataTransferItemList back into an interface class
https://bugs.webkit.org/show_bug.cgi?id=76856

When Qt implemented the DataTransferItemList, a lot of logic was moved into the shared
classes since Chromium/Qt happened to implement it the same way. Now that I want to do some
refactoring/cleanup work to better implement DataTransferItemList in Chromium, we won't
share the same data anymore so it doesn't make sense to keep that code in a common location.

Reviewed by David Levin.

Covered by existing tests.

* CMakeLists.txt:
* GNUmakefile.list.am:
* Target.pri:
* WebCore.gypi:
* WebCore.vcproj/WebCore.vcproj:
* WebCore.xcodeproj/project.pbxproj:
* dom/DataTransferItem.cpp:
* dom/DataTransferItem.h:
* dom/DataTransferItemList.cpp: Removed.
* dom/DataTransferItemList.h:
* platform/chromium/ClipboardChromium.cpp:
(WebCore::ClipboardChromium::mayUpdateItems):
* platform/chromium/DataTransferItemChromium.cpp:
(WebCore::DataTransferItemChromium::create):
(WebCore::DataTransferItemChromium::DataTransferItemChromium):
(WebCore::DataTransferItemChromium::getAsString):
(WebCore::DataTransferItemChromium::getAsFile):
(WebCore::DataTransferItemChromium::clipboardChromium):
* platform/chromium/DataTransferItemChromium.h:
(WebCore::DataTransferItemChromium::kind):
(WebCore::DataTransferItemChromium::type):
* platform/chromium/DataTransferItemListChromium.cpp:
(WebCore::DataTransferItemListChromium::DataTransferItemListChromium):
* platform/chromium/DataTransferItemListChromium.h:
* platform/qt/DataTransferItemListQt.cpp:
(WebCore::DataTransferItemListQt::DataTransferItemListQt):
(WebCore::DataTransferItemListQt::length):
(WebCore::DataTransferItemListQt::item):
(WebCore::DataTransferItemListQt::deleteItem):
(WebCore::DataTransferItemListQt::clear):
(WebCore::DataTransferItemListQt::add):
* platform/qt/DataTransferItemListQt.h:
* platform/qt/DataTransferItemQt.cpp:
(WebCore::DataTransferItemQt::create):
(WebCore::DataTransferItemQt::DataTransferItemQt):
(WebCore::DataTransferItemQt::getAsString):
(WebCore::DataTransferItemQt::getAsFile):
* platform/qt/DataTransferItemQt.h:
(WebCore::DataTransferItemQt::kind):
(WebCore::DataTransferItemQt::type):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105648 268f45cc-cd09-0410-ab3c-d52691b4dbfc