Fix drag/drop on Chrome by setting an image src

Work around a Chrome regression where it aborts the drag if the
setDragImage image is src-less.

Closes codemirror#1810

lib/codemirror.js

@@ -1912,6 +1912,7 @@ window.CodeMirror = (function() {
     // Recent Safari (~6.0.2) have a tendency to segfault when this happens, so we don't do it there.
     if (e.dataTransfer.setDragImage && !safari) {
       var img = elt("img", null, null, "position: fixed; left: 0; top: 0;");
+      img.src = "data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==";
       if (opera) {
         img.width = img.height = 1;
         cm.display.wrapper.appendChild(img);