Clear the suggested value while setting value for <select> element.

R=isherman@chromium.org, tkent@chromium.org
BUG=380603

Review URL: https://codereview.chromium.org/319443003

git-svn-id: svn://svn.chromium.org/blink/trunk@175768 bbb929c8-8fbe-4397-9dbb-9b2b20218538