Pass rootNode by reference when constructing node lists / collections

Pass rootNode by reference when constructing node lists / collections to make
it clear that the rootNode cannot be null. Their implementation already relies
on the fact that the rootNode cannot be null.

R=adamk@chromium.org, adamk

Review URL: https://codereview.chromium.org/176933013

git-svn-id: svn://svn.chromium.org/blink/trunk@168417 bbb929c8-8fbe-4397-9dbb-9b2b20218538