Move concept of PostAttachCallbacks out of ContainerNode

We no longer need to suspend post attach callbacks or widget hierarchy updates as we go
down through Element::attach since it only happens inside ::recalcStyle and
Document::recalcStyle already does this at the top of the stack. After removing those two
calls inside Element::attach I realized we could refactor the post attach callback handling
into a separate thing that Document uses as well.

BUG=276689

Review URL: https://chromiumcodereview.appspot.com/23537031

git-svn-id: svn://svn.chromium.org/blink/trunk@157503 bbb929c8-8fbe-4397-9dbb-9b2b20218538