Have LiveNodeListBase::ownerNode() return a reference

Have LiveNodeListBase::ownerNode() return a reference instead of a pointer as
it can never be null. It makes the code look safer, no longer requires
assertions and avoids unnecessary null checks in the generated bindings.

R=adamk@chromium.org, haraken@chromium.org, adamk, haraken
NOTRY=true

Review URL: https://codereview.chromium.org/180143003

git-svn-id: svn://svn.chromium.org/blink/trunk@167848 bbb929c8-8fbe-4397-9dbb-9b2b20218538