Move DropTargetEvent back to ui/base for the time being as it depends on OSExchangeData which I don't want to move.

BUG=none
R=sky@chromium.org
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/23503086

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225019 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225019