Remove containsAttributeSelector() checks.

We can just set the style as unique from inside the SelectorChecker
instead. That's not ideal either, but it's simpler, and we'll eventually
use a hashing function to instead make elements with attribute selectors
that have the same values share their style instead of disabling
sharing.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/803113004