Devirtualize the Clipboard implementation.

Similar to the DataTransferItem/DataTransferItemList patch, there's no
need to separate the non-Chromium and Chromium parts, since we can just
use the platform abstraction directly.

Other incidental cleanups:
- Clipboard no longer inherits from CachedImageClient.
- m_dragStarted was unused and removed

BUG=298987
R=tony@chromium.org

Review URL: https://codereview.chromium.org/24809003

git-svn-id: svn://svn.chromium.org/blink/trunk@158409 bbb929c8-8fbe-4397-9dbb-9b2b20218538