HTMLPlugInElement should not use confusingAndOftenMisusedAttached()

This if statement didn't make any sense. If you have a renderer then
you're certainly attached so checking confusingAndOftenMisusedAttached()
and renderer() was silly. Simplify the check and remove the confusing
comment since that doesn't seem to be true anymore.

I also added a FIXME since all this code related to widget updates looks
wrong.

Review URL: https://codereview.chromium.org/50163004

git-svn-id: svn://svn.chromium.org/blink/trunk@160883 bbb929c8-8fbe-4397-9dbb-9b2b20218538