Always Reconstruct when stylesheets change.

In preparation of simplifying how we collect and process sheets now
that we don't support descendant, sibling and tree boundary crossing
rules we should switch to always going down the Reconstruct path.

This might be somewhat slower to start, but because our rules are so
much simpler we should be able to rebuild the StyleEngine to do
something simpler and faster.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/774953002