Teach ScopedStyleResolver that it's owned by a TreeScope.

Soon we're going to make ScopedStyleResolver be owned by the TreeScope
itself, for now lets make its scope pointer be to a TreeScope so code
know it's never a random element.

BUG=392018

Review URL: https://codereview.chromium.org/372103007

git-svn-id: svn://svn.chromium.org/blink/trunk@177634 bbb929c8-8fbe-4397-9dbb-9b2b20218538