Add HTMLFormControlElement::supportsAutofocus to fix a FIXME comment.

This virtual function should return true if the form control can hanlde
'autofocucs' attribute if it is specified.

Note: HTMLInputElement::supportsAutofocus reuses InputType::isInteractiveContent
because interactiveness is required for autofocus capability.

BUG=none
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/143343003

git-svn-id: svn://svn.chromium.org/blink/trunk@165432 bbb929c8-8fbe-4397-9dbb-9b2b20218538
void HTMLSelectElement::setMultiple(bool multiple)
{
    bool oldMultiple = this->multiple();
    int oldSelectedIndex = selectedIndex();
    setAttribute(multipleAttr, multiple ? emptyAtom : nullAtom);

    if (oldMultiple != this->multiple())
        setSelectedIndex(oldSelectedIndex);
}

void HTMLSelectElement::setMultiple(bool multiple)
{
    bool oldMultiple = this->multiple();
    int oldSelectedIndex = selectedIndex();
    setAttribute(multipleAttr, multiple ? emptyAtom : nullAtom);

    if (oldMultiple != this->multiple())
        setSelectedIndex(oldSelectedIndex);
}
