StyleSheetContents::clearResolvers should clear ScopedStyleResolvers

This is a fix for crashes in oilpan builds after landing https://codereview.chromium.org/415743005/.

StyleSheetContents::clearResolvers should clear ScopedStyleResolvers as well as normal StyleResolvers.
The original CL (r178795) that introduced ScopedStyleResolver was missing this code,
and it exposed the bug in oilpan builds when we landed r178817.

BUG=340522

Review URL: https://codereview.chromium.org/419513002

git-svn-id: svn://svn.chromium.org/blink/trunk@178840 bbb929c8-8fbe-4397-9dbb-9b2b20218538