Using alt and down key to open datalist using keyboard

On pressing alty+down key, datalist is open and no stepDown is generated.

R=tkent@chromium.org, isherman@chromium.org, keishi1@chromium.org
BUG=229073
TEST= On down+alt key combination does not generate stepDown.

Review URL: https://codereview.chromium.org/218683010

git-svn-id: svn://svn.chromium.org/blink/trunk@170673 bbb929c8-8fbe-4397-9dbb-9b2b20218538