Reduce use of downcast<>() in DataTransfer.cpp, for performance.
Pull request: https://github.com/WebKit/WebKit/pull/25685
Committed 275894@main (fd1434216dd2): <https://commits.webkit.org/275894@main> Reviewed commits have been landed. Closing PR #25685 and removing active labels.
<rdar://problem/124354857>