Gracefully handle drag cancellation during drag start.

Should the dispatching of a dragstart cause the drag to be reset, take
that into account when trying to handle follow-on drag operations.

The test is imported from https://bugs.webkit.org/show_bug.cgi?id=115296

R=dcheng@chromium.org
BUG=405442

Review URL: https://codereview.chromium.org/488923003

git-svn-id: svn://svn.chromium.org/blink/trunk@180646 bbb929c8-8fbe-4397-9dbb-9b2b20218538