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This is because DraggableButton inherits from NSButton when a future class will
need it to inherit from NSPopupButton. Because ObjC lacks multiple inheritance,
and we want to avoid runtime trickery, this is the result.

BUG=none
TEST=Dragging download shelf and bookmark bar items works as before. Partially covered by unit tests.

Review URL: http://codereview.chromium.org/7462018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@96433 0039d316-1c4b-4281-b951-d872f2087c98