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 HTMLTextAreaElement::updateFocusAppearance(bool restorePreviousSelection)
{
    if (!restorePreviousSelection || !hasCachedSelection()) {
        setSelectionRange(0, 0);
    } else
        restoreCachedSelection();

    if (document().frame())
        document().frame()->selection().revealSelection();
}

void HTMLTextAreaElement::updateFocusAppearance(bool restorePreviousSelection)
{
    if (!restorePreviousSelection || !hasCachedSelection()) {
        setSelectionRange(0, 0);
    } else
        restoreCachedSelection();

    if (document().frame())
        document().frame()->selection().revealSelection();
}
