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 HTMLKeygenElement::parseAttribute(const QualifiedName& name, const AtomicString& value)
{
    if (name == disabledAttr)
        shadowSelect()->setAttribute(name, value);

    HTMLFormControlElement::parseAttribute(name, value);
}

void HTMLKeygenElement::parseAttribute(const QualifiedName& name, const AtomicString& value)
{
    if (name == disabledAttr)
        shadowSelect()->setAttribute(name, value);

    HTMLFormControlElement::parseAttribute(name, value);
}
