Move LiveNodeListBase code out of HTMLCollection.cpp

Move LiveNodeListBase code out of HTMLCollection.cpp and into
LiveNodeListBase.h.

R=adamk@chromium.org, adamk, esprehn

Review URL: https://codereview.chromium.org/180383002

git-svn-id: svn://svn.chromium.org/blink/trunk@167847 bbb929c8-8fbe-4397-9dbb-9b2b20218538