Popup open and hide using Alt+KeyDown

HTMLSelect to open popup when key combination alt+keydown. It does not select any element, just opens or hide popup.

Once popup is open, default handler of the popup handles key event. For that reason hide popup is handled in PopupListBox.

This behavior is compatible with firefox behavior.

R=tkent, keishi
BUG=383818
TEST=On Alt+keydown open popup and check if visible. On again press of Alt+keydown, close popup.

Review URL: https://codereview.chromium.org/368023003

git-svn-id: svn://svn.chromium.org/blink/trunk@177834 bbb929c8-8fbe-4397-9dbb-9b2b20218538