hide-if-contains snippet doesn't observe attributes changes

Background

hide-if-contains snippet doesn't observe attributes changes. But if we use a selector that select on attributes, this might go around the observer.

The upcoming hide-if-contains-visible-text sharing the code is impacted too. (Trac 7540)

What to change

Evaluate if this is necessary and the impact (on performance). If it is worth it, change the snippet code in lib/content/snippets.js to observe attribute changes.