Move Clipboard-related code from core/dom to core/clipboard

A lot of this stuff landed in dom when it was ousted from core/platform; most of
these classes would be happier in blink_platform (save for Clipboard,
DataTransferItem, and DataTransferItemList, which all have associated IDL files),
but due to a few File dependencies it's not trivial to move there.

But since it doesn't belong in dom (they're not related to Document structure),
a new clipboard directory seems the best bet.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/153343003

git-svn-id: svn://svn.chromium.org/blink/trunk@166738 bbb929c8-8fbe-4397-9dbb-9b2b20218538