Prevents a node in user agent shadow trees from be drag-and-dropped from outside of shadow trees.

r178285 made drag and drop work for a node in shadow trees, however,
it turned out that built-in elements, such as <input> elements, have a custom drag and drop behavior.
This patch reverts the drag-and-drop capability only for a node in user agent shadow trees.

BUG=417170
TEST=new layout test and manual test for the case reported in the bug

Review URL: https://codereview.chromium.org/608683002

git-svn-id: svn://svn.chromium.org/blink/trunk@182880 bbb929c8-8fbe-4397-9dbb-9b2b20218538