Prevent leak on completion of drop operation.

If the 'drop' event handler signals that the event has been handled and
no default handling is to be done, we clear the document and consider
the operation complete. Also clear out the drag caret selection at that
time, as it may hold onto an anchor node (and its document.)

R=dcheng
BUG=364417
TEST=fast/dom/shadow/drop-event-for-input-in-shadow.html

Review URL: https://codereview.chromium.org/689103003

git-svn-id: svn://svn.chromium.org/blink/trunk@184768 bbb929c8-8fbe-4397-9dbb-9b2b20218538