Move ColorChooser{,Client} and DateTimeChooser{,Client} from platform to core.

We're going to add |Element& ownerElement()| to ColorChooserClient
and DateTimeChooserClient, and add |AXObject* rootAXObject()| to
ColorChooser and DateTimeChooser.
Classes in platform/ can't refer to core classes such as Element and
AXObject. So we move these classes.

BUG=123896

Review URL: https://codereview.chromium.org/512563002

git-svn-id: svn://svn.chromium.org/blink/trunk@180934 bbb929c8-8fbe-4397-9dbb-9b2b20218538