Automate ManualTests/drag-image-to-desktop.html

Make drag-image-to-desktop a http/tests/images test, and use EventSender's
dumpFilenameBeingDragged() to examine the drag-drop files.

The manual test was added in https://bugs.webkit.org/show_bug.cgi?id=20685
which pre-dates dumpFilenameBeingDragged() addition to DRT.

BUG=259669

Review URL: https://chromiumcodereview.appspot.com/19672015

git-svn-id: svn://svn.chromium.org/blink/trunk@154806 bbb929c8-8fbe-4397-9dbb-9b2b20218538