Some Scrollbar functions assume an attached ScrollableArea but can be called without one
https://bugs.webkit.org/show_bug.cgi?id=54262

Reviewed by Dan Bernstein.

Make sure that all calls to the scrollable area are null checked.

* platform/Scrollbar.cpp:
(WebCore::Scrollbar::autoscrollPressedPart):
(WebCore::Scrollbar::moveThumb):
(WebCore::Scrollbar::mouseMoved):



git-svn-id: svn://svn.chromium.org/blink/trunk@78431 bbb929c8-8fbe-4397-9dbb-9b2b20218538
void Scrollbar::autoscrollTimerFired(Timer<Scrollbar>*)
{
    autoscrollPressedPart(theme()->autoscrollTimerDelay());
}

void Scrollbar::autoscrollTimerFired(Timer<Scrollbar>*)
{
    autoscrollPressedPart(theme()->autoscrollTimerDelay());
}
