Merge ChildNodeInsertionNotifier and ChildNodeRemovalNotifier into ContainerNode

We don't really need these classes anymore, we can just use methods on
ContainerNode. This gets us one step closer to completely eliminating
ContainerNodeAlgorithms which now only contains one shared method related
to SVGElementInstance and cleanup code that Oilpan will eliminate.

Review URL: https://codereview.chromium.org/305603003

git-svn-id: svn://svn.chromium.org/blink/trunk@174982 bbb929c8-8fbe-4397-9dbb-9b2b20218538