Remove ContainerNodeAlgorithms

With Rob Buis's epic removal of SVGElementInstance(crbug.com/313438),
the strange ContainerNodeAlgorithms abstraction can be folded into
ContainerNode.

For reviewability, this patch makes only the minimal changes needed to
merge these files.

BUG=392970

Review URL: https://codereview.chromium.org/382013005

git-svn-id: svn://svn.chromium.org/blink/trunk@177888 bbb929c8-8fbe-4397-9dbb-9b2b20218538