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 HTMLFormControlElement::willCallDefaultEventHandler(const Event& event)
{
    if (!event.isKeyboardEvent() || event.type() != EventTypeNames::keydown)
        return;
    if (!m_wasFocusedByMouse)
        return;
    m_wasFocusedByMouse = false;
    if (renderer())
        renderer()->repaint();
}

void HTMLFormControlElement::willCallDefaultEventHandler(const Event& event)
{
    if (!event.isKeyboardEvent() || event.type() != EventTypeNames::keydown)
        return;
    if (!m_wasFocusedByMouse)
        return;
    m_wasFocusedByMouse = false;
    if (renderer())
        renderer()->repaint();
}
