Simplify styledSubtreeSize

This function can be made a lot simpler, and iterative instead of
recursive. We don't really need a ShadowRoot aware traversal method,
you can easily just travserse the tree normally and recurse on
ShadowRoots when needed.

R=eseidel@chromium.org,ojan@chromium.org

Review URL: https://codereview.chromium.org/284063002

git-svn-id: svn://svn.chromium.org/blink/trunk@174073 bbb929c8-8fbe-4397-9dbb-9b2b20218538