Use auto and Range-based loop interation in Source/core/dom/custom/

This patch adds combined usage of auto and Range-based loop
iteration as per allowed C++11 features.

Review URL: https://codereview.chromium.org/610273005

git-svn-id: svn://svn.chromium.org/blink/trunk@182927 bbb929c8-8fbe-4397-9dbb-9b2b20218538