[chromium] Refactor Clipboard invalidate for DataTransferItem/DataTransferItemList...

[chromium] Refactor Clipboard invalidate for DataTransferItem/DataTransferItemList into a wrapper
https://bugs.webkit.org/show_bug.cgi?id=76993

We want to unify the backing data store for ClipboardChromium and DataTransferItems. For
that, we want use a similar representation to DataTransferItem list inside
ChromiumDataObject.  However, since ChromiumDataObject should be valid in scopes where
Clipboard is not (e.g. default drag processing), we need to separate the clipboard
invalidation logic into a wrapper class.

Reviewed by Tony Chang.

Covered by existing tests.

* platform/chromium/ClipboardChromium.cpp:
():
(WebCore::ClipboardChromium::items):
* platform/chromium/DataTransferItemChromium.cpp:
(WebCore::DataTransferItemChromium::getAsString):
* platform/chromium/DataTransferItemListChromium.cpp:
(WebCore::DataTransferItemListChromium::length):
(WebCore::DataTransferItemListChromium::item):
(WebCore::DataTransferItemListChromium::deleteItem):
(WebCore::DataTransferItemListChromium::clear):
(WebCore::DataTransferItemListChromium::add):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105928 268f45cc-cd09-0410-ab3c-d52691b4dbfc