ImageInputType::ensurePrimaryContent should recreate UA shadow tree.

Once the fallback shadow tree was created, it was never recreated even if
ensurePrimaryContent was called.  Such situation happens by updating |src|
attribute.

BUG=589838

Review URL: https://codereview.chromium.org/1732753004

Cr-Commit-Position: refs/heads/master@{#377804}
static IntPoint extractClickLocation(Event* event)
{
    if (!event->underlyingEvent() || !event->underlyingEvent()->isMouseEvent())
        return IntPoint();
    MouseEvent* mouseEvent = toMouseEvent(event->underlyingEvent());
    if (!mouseEvent->hasPosition())
        return IntPoint();
    return IntPoint(mouseEvent->offsetX(), mouseEvent->offsetY());
}

static IntPoint extractClickLocation(Event* event)
{
    if (!event->underlyingEvent() || !event->underlyingEvent()->isMouseEvent())
        return IntPoint();
    MouseEvent* mouseEvent = toMouseEvent(event->underlyingEvent());
    if (!mouseEvent->hasPosition())
        return IntPoint();
    return IntPoint(mouseEvent->offsetX(), mouseEvent->offsetY());
}
