Move statics from ScopeContentDistribution to ShadowRoot

ScopeContentDistribution contains a bunch of static methods that entirely
delegate to ShadowRoot and should just be methods there. This moves them
to their correct home.

Review URL: https://chromiumcodereview.appspot.com/19961002

git-svn-id: svn://svn.chromium.org/blink/trunk@154699 bbb929c8-8fbe-4397-9dbb-9b2b20218538