Make CTBuilderList its own class

...in preparation for https://codereview.chromium.org/502293003/

R=ojan@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/522043002

git-svn-id: svn://svn.chromium.org/blink/trunk@181267 bbb929c8-8fbe-4397-9dbb-9b2b20218538