Remove style element counting in ShadowRoot.

This optimization doesn't make sense, all it does is avoid adding
the ScopedStyleResolver for the ShadowRoot when looking for host
styles, but having that extra resolver shouldn't be slow, and in
sky it's very rare to have a ShadowRoot without styles unlike
on the web where <input> is very common and gets its style from
the UA instead of from a <style> like a normal component.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/797873002