Cleanup: remove ScopedStyleResolver::scopingNode(), use TreeScope instead

No functional change.

BUG=none
TEST=pass layout tests.

Review URL: https://codereview.chromium.org/448073002

git-svn-id: svn://svn.chromium.org/blink/trunk@179778 bbb929c8-8fbe-4397-9dbb-9b2b20218538