Have getChildNodes() take a ContainerNode in argument

Have getChildNodes() take a ContainerNode in argument instead of a Node as
this utility function only makes sense for ContainerNodes. Also, this lets
the compiler generate slighter more efficient code.

This CL also makes use of getChildNodes() in more places to avoid code
duplication.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/435733002

git-svn-id: svn://svn.chromium.org/blink/trunk@179356 bbb929c8-8fbe-4397-9dbb-9b2b20218538