Remove all members in ScopedStyleTree for preparing class removal

As a part of ScopedStyleTree removal (issue 392018),
remove all private members in ScopedStyleTree.

|m_cache| is no longer necessary as ScopedStyleResolver
lookup from an element no longer require ascending the
DOM tree up, thanks to <style scoped> removal.

Unfortunately |m_authorStyles| cannot be removed completely
- StyleResolver still has to clear ScopedStyleResolvers
which are associated with TreeScopes, and thus moved from
ScopedStyleTree to StyleResolver (and renamed
|m_scopedStyleResolvers| to match what it contains).

BUG=392018
TEST=no layout test failures

Review URL: https://codereview.chromium.org/418833002

git-svn-id: svn://svn.chromium.org/blink/trunk@179051 bbb929c8-8fbe-4397-9dbb-9b2b20218538