Get rid of StyleTreeScopeTracker

It turns out the class, which was introduced at r162439, isn't worth
having. It has dependency to StyleEngine thus we cannot say it is
well factored out. This change flattens the class back into StyleEngine.

BUG=none
TEST=none
R=tasak,hayato

Review URL: https://codereview.chromium.org/104833004

git-svn-id: svn://svn.chromium.org/blink/trunk@163934 bbb929c8-8fbe-4397-9dbb-9b2b20218538