SelectElement should remove an option when null is assigned by indexed setter

Fix bug embedded in r151449

see
http://src.chromium.org/viewvc/blink?revision=151449&view=revision

R=haraken@chromium.org, tkent@chromium.org, eseidel@chromium.org
BUG=262365
TEST=fast/forms/select/select-assign-null.html

Review URL: https://chromiumcodereview.appspot.com/19947008

git-svn-id: svn://svn.chromium.org/blink/trunk@154743 bbb929c8-8fbe-4397-9dbb-9b2b20218538