GitLab

This reverts 7d6c8b29, which made window server have access to the
clipboard, as at the time, I thought that drag and drop would read/write
to/from a dedicated drag clipboard and that the window server would have
to manipulate it. Due to timing issues, this didn't work out.

Now that we aren't using a drag clipboard as the backing for mus drag
and drop, also remove the drag clipboard from our mus clipboard
implementation.

BUG=614037

Review-Url: https://codereview.chromium.org/2339353003
Cr-Commit-Position: refs/heads/master@{#419260}