Problem: JS 'focus' handler fired by drag-n-drop can't restore input value.

Solution: enqueue event dispatching, so handlers execute when state is stable.

BUG=263743

Review URL: https://codereview.chromium.org/21030005

git-svn-id: svn://svn.chromium.org/blink/trunk@158811 bbb929c8-8fbe-4397-9dbb-9b2b20218538