[cssom] Define style sheet lifetime better

[annevk]

There are a number of issues with HTML and CSS go about creating style sheets and managing them. There's a couple I know from looking into this a tiny bit and I suspect that anyone looking into this more will find many more.

• The link or style element being removed does not cause the style sheet to be removed. (It is only removed if HTML sees reason to create a new style sheet.
• It's not clear at what point .sheet starts pointing at an object.

Some issues here might be better addressed at the HTML side (if they don't apply more generally as HTML is not the only way to create style sheets), but I think before that it would help if CSSOM made the infrastructure here a little less brittle, defined the arguments better for the various algorithms, etc.