interactive-graph: enable trashcan translucency

Summary:
We had to disable this in Chrome 34, since Chrome had a bug that
wouldn't render some of the svg when there was a translucent image.

This bug appears to be gone now, so we can bring back this
functionality and have a better UI!

Test Plan:
 * Make an interactive graph, with click-to-add-points mode for both
   points and polygons
 * Try dragging the points around
 * See that the trashcan becomes opaque only while dragging
 * See that the other points and the polygon are always visible
   and reasonably displayed (no weird stub lines to nowhere)

Auditors: charlie, jared