Have ScopedStyleResolver deal with ContainerNode references, not pointers

Have ScopedStyleResolver deal with ContainerNode references, not pointers as
it can never be NULL. This is now obvious to callers that the ContainerNode
cannot be NULL.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23567016

git-svn-id: svn://svn.chromium.org/blink/trunk@157090 bbb929c8-8fbe-4397-9dbb-9b2b20218538