Remove support for legacy WebNodeCollection

Remove support for legacy WebNodeCollection now that all the callers on
Chromium side have been updated to use WebElementCollection instead.

R=tkent, dglazkov@chromium.org

Review URL: https://codereview.chromium.org/145353019

git-svn-id: svn://svn.chromium.org/blink/trunk@166939 bbb929c8-8fbe-4397-9dbb-9b2b20218538