Only show tree status if the tree isn't open.

In the spirit of sheriff-o-matic only showing problems that
need solving, there's no point in showing tree open status.
We might want to reconsider this in the future where we
show you the actual status message, but right now the open
state is hard-coded to say "Tree is open", which is not
information I need to see.

Also, cleaned up a couple minor polymer things:
-No need to set the attributes attribute if we're using publish.
-Initialize all the member variables on ct-tree-status.
-Don't use setAttribute. Should basically always set element
values by property.

NOTRY=true
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/403723005

git-svn-id: svn://svn.chromium.org/blink/trunk@178524 bbb929c8-8fbe-4397-9dbb-9b2b20218538