When in the middle of a drag & drop session in the browser (bookmarks bar, omnibox text, extensions, etc), a red circle-with-line-through-it will be overlaid on the drag target if dropping it on the current location is not possible, and a green "+" will be overlaid if dropping it is possible.

Some updates needed:

(1) With touch d&d at a 2x scale factor, these icons look blurry. (With mouse they are drawn at a smaller size and look OK at both 1x and 2x.)

(2) The icons themselves look a bit outdated, i.e., I don't think they were considered as part of the top chrome MD update.

Consider providing new MD-style icons in SVG format so they can be vectorized. Alternately, we could consider doing away with the icons altogether and just shading the background of the thing being dragged as green/red.