Remove ScopedStyleTree

After r179051, state is no longer kept in ScopedStyleTree and
all its methods can be somewhere else.

Merge all methods in ScopedStyleTree in StyleResolver and remove
ScopedStyleTree class.

BUG=392018
TEST=no layout test failures.

Review URL: https://codereview.chromium.org/424963002

git-svn-id: svn://svn.chromium.org/blink/trunk@179212 bbb929c8-8fbe-4397-9dbb-9b2b20218538