Rename Clipboard to DataTransfer.

WebCore::Clipboard implements the DataTransfer interface defined in the
Drag and Drop and Clipboard standards. This change renames it to
WebCore::DataTransfer, to reduce cognitive overhead when reading the
code that uses it.

Review URL: https://codereview.chromium.org/356673002

git-svn-id: svn://svn.chromium.org/blink/trunk@177011 bbb929c8-8fbe-4397-9dbb-9b2b20218538