Propagate onInput event after changing the HTMLSelect value

Dispatch input event when the value of the <select> element is
modified.

R=tkent@chromium.org, mkwst@chromium.org
BUG=349472

Review URL: https://codereview.chromium.org/187923004

git-svn-id: svn://svn.chromium.org/blink/trunk@169047 bbb929c8-8fbe-4397-9dbb-9b2b20218538