Use more const references in HTMLCollection / LiveNodeList

The code looks safer as it is now obvious the nodes / lists cannot
be null. Also, making them const also makes the code look safer as
we know the nodes are not altered by the methods.

R=esprehn, adamk, eseidel

Review URL: https://codereview.chromium.org/141683004

git-svn-id: svn://svn.chromium.org/blink/trunk@165504 bbb929c8-8fbe-4397-9dbb-9b2b20218538